Open Access

Sequencing of bovine herpesvirus 4 v.test strain reveals important genome features

  • Leonor Palmeira1,
  • Bénédicte Machiels1,
  • Céline Lété1,
  • Alain Vanderplasschen1 and
  • Laurent Gillet1Email author
Virology Journal20118:406

DOI: 10.1186/1743-422X-8-406

Received: 17 June 2011

Accepted: 16 August 2011

Published: 16 August 2011

Abstract

Background

Bovine herpesvirus 4 (BoHV-4) is a useful model for the human pathogenic gammaherpesviruses Epstein-Barr virus and Kaposi's Sarcoma-associated Herpesvirus. Although genome manipulations of this virus have been greatly facilitated by the cloning of the BoHV-4 V.test strain as a Bacterial Artificial Chromosome (BAC), the lack of a complete genome sequence for this strain limits its experimental use.

Methods

In this study, we have determined the complete sequence of BoHV-4 V.test strain by a pyrosequencing approach.

Results

The long unique coding region (LUR) consists of 108,241 bp encoding at least 79 open reading frames and is flanked by several polyrepetitive DNA units (prDNA). As previously suggested, we showed that the prDNA unit located at the left prDNA-LUR junction (prDNA-G) differs from the other prDNA units (prDNA-inner). Namely, the prDNA-G unit lacks the conserved pac-2 cleavage and packaging signal in its right terminal region. Based on the mechanisms of cleavage and packaging of herpesvirus genomes, this feature implies that only genomes bearing left and right end prDNA units are encapsulated into virions.

Conclusions

In this study, we have determined the complete genome sequence of the BAC-cloned BoHV-4 V.test strain and identified genome organization features that could be important in other herpesviruses.

Background

Gammaherpesviruses are archetypal persistent viruses which are ubiquitous in both human and animal populations. The human gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi's Sarcoma-associated Herpesvirus (KSHV), infect respectively some 90% [1] and 30% [2] of human populations and cause several cancers [2, 3]. Although much effort has been invested on these viruses, studies of EBV or KSHV are difficult to perform directly because these viruses show limited lytic growth in vitro and have no well-established in vivo infection model. Related animal gammaherpesviruses are therefore an important source of information.

Bovine herpesvirus 4 (BoHV-4) belongs to the Gammaherpesvirinae subfamily, and to the Rhadinovirus genus [4]. Similarly to its human counterparts, BoHV-4 was found to be widespread in all bovine populations and to persist in the vast majority of individuals as a lifelong, asymptomatic infection [5]. However, in some circumstances, BoHV-4 has been associated with various clinical symptoms such as skin lesions, respiratory diseases, metritis, malignant catarrhal fever or tumors [5].

The virus was first isolated in Europe by Bartha et al. from calves with respiratory diseases [6] and later in North America by Mohanty et al. [7]. Besides cattle, BoHV-4 has also been detected in a variety of ruminants. In particular, BoHV-4 seems to be highly prevalent among wild African buffalo (Syncerus caffer) which could be considered as the natural reservoir of the virus [810]. Overall, more than 40 BoHV-4 strains have been isolated across the world. These strains can be classified in three groups: the European strains (or Movar 33/63-like strains), the American strains (or DN 599-like strains) and the African buffalo strains [9].

It is estimated that the taurine and buffalo strains diverged around 730,000 years ago [9] and that the European and North American clades diverged around 260,000 years ago [9]. The genome of the BoHV-4 66-p-347 North American strain has entirely been sequenced [11]. However, the BAC-cloned reference strain V.test [12, 13] belongs to the European clade [9, 14]. Previous studies suggested that the BoHV-4 V-test strain contains regions of high dissimilarity compared to the BoHV-4 66-p-347 strain. Indeed, the nucleotide identity between the two strains has been previously measured to be as low as 88% on the BORFB2 region [11]. However, the lack of a complete genomic sequence for the V.test strain prevents from drawing a general view concerning this divergence level. Therefore, the low quality of the genomic information hampers the use of the BAC-cloned BoHV-4 V.test strain as a good model for studying gammaherpesvirus biology. In this study, we have determined the genomic sequence of the BoHV-4 V.test strain and analyzed its overall differences with the available sequence of the BoHV-4 66-p-347 strain [11, 15]. The results obtained highlighted important differences between BoHV-4 66-p-347 and V.test strains. Moreover complete sequencing of the BoHV-4 V.test strain also revealed genome features potentially important in other herpesviruses.

Methods

BAC sequencing

BAC DNA was purified using Qiagen large-construct kit as described by the manufacturer. The complete BAC cloned viral genome of BoHV-4 V.test strain was determined by pyrosequencing using the 454 GS FLX Titanium (Roche) high-throughput sequencer and resulted in 48,967 reads of an average read length of 265 nucleotides and a total of 12,997,275 bases. A targeted ABI-Sanger sequencing of fragments of the prDNA region was also conducted using the primers listed in Table 1. The raw 454 data has been deposited in the NCBI Sequence Read Archive (SRA) database with accession number SRA037246.
Table 1

Primers used in this study

name

Sequence

Coordinates according to Genbank

Bo1 Fwd

5'- ATGGAGGGTGATGGATTCATG-3'

460-440a

Bo1 Rev

5'- TTAAGGCCTCATTCCAGGAAG-3'

272-292a

Bo5 Fwd

5'- GCTACAGAAAATGGCCAGTAAAG-3'

20366-20342a

Bo5 Rev

5'- TCATGTCCTGAGTGGGTCTATG-3'

19170-19191a

Bo6 Fwd

5'- ATGGTCATCCTAAATGCTCAAG -3'

20297-20318a

Bo6 Rev

5'- TCACCTAGTGTTGCAACCCC -3'

20497-20478a

Bo7 Fwd

5'- ATGGAGACAATTTCCATAAACTG -3'

20994-20972a

Bo7 Rev

5'- CTAGCTGGGGTAGAGTGATC -3'

20671-20690a

ORF67.5 Fwd

5'- ATGGCTGATGGTGATGTTTTAG -3'

93144-93123a

ORF67.5 Rev

5'- TCAATGTTTGTCCAGAGCACT -3'

92881-92901a

Bo12 Fwd

5'- ATGGGGGCGCTATTTGGGC -3'

97442-97460a

Bo12 Rev

5'- TCAACTGATGAAACCCACCC -3'

97525-97506a

Bo13 Fwd

5'- ATGCGTCTCGATGGCAAGC -3'

98838-98856a

Bo13 Rev

5'- CTATGGTTGTTTTTTAAAGAAAATC -3'

98981-98957a

ORF75 Fwd

5'- ATGTATCCCAGATACAGTAACA -3'

103606-103585a

ORF75 Rev

5'- TTACATTTTATTTTTCAGACACCA -3'

100274-100297a

prDNA Fwd 1

5'- GGAGCCCAAAACCAAAAGAG -3'

870-889b

prDNA Rev 1

5'- CTCTTTTGGTTTTGGGCTCC -3'

889-870b

prDNA Fwd 2

5'- CGTAGGCCTCACATTCAGC -3'

908-926b

prDNA Rev 2

5'- GCTGAATGTGAGGCCTACG -3'

926-908b

prDNA Fwd 3

5'- CGAGAGATGGTTCTTGCACA -3'

940-959b

prDNA Rev 3

5'- TGTGCAAGAACCATCTCTCG -3'

959-940b

BAC Rev

5'- TTGCCAATCCCAAAAAGAAG -3'

9859-9878c

a according to Genbank JN133502 sequence (BoHV-4 V.test strain long unique region)

b according to Genbank JN133504 sequence (BoHV-4 V.test strain prDNA inner region)

c according to Genbank AY665170.1 sequence (pBeloBAC modified)

BoHV-4 genome LUR assembly

The reads were de novo assembled with gsAssembler (Roche), where the E. coli genome was used as a contaminant to filter out cellular reads [16]. The filtering removed 1,167 contaminant cellular reads. The de novo assembly yielded 11 contigs which were subsequently BLASTed against 66-p-347's long unique region (LUR) and polyrepetitive DNA (prDNA) -accession numbers NC_002665 and AF092919- to define their relative positions [17]. Contigs were assembled into a large scaffold using two previously published V.test sequences (accession numbers Z46380 and Z46385 [18]) overlapping contig borders. A careful comparison of the bordering contigs with the previously sequenced fragments showed a high percent identity (> 99.99%). After verification of the quality of the assembly, the BAC sequence was removed and the genome sequence was annotated as detailed hereunder.

BoHV-4 genome prDNA assembly

The prDNA was determined by a hybrid 454/ABI-Sanger strategy where 17 ABI-Sanger fragments of prDNA were de novo assembled with the 454 reads. Briefly, in order to correctly assemble the prDNA and to disentangle different prDNA units, this second de novo assembly was optimized for highly repetitive segments using MIRA [19]. 454 reads and quality information were extracted from the raw .sff file with 'sff_extract'. The base-calling and quality-calling for Sanger sequences were inferred from the .ab1 raw chromatogram files using 'phred' [20, 21] and the sequences were quality-trimmed using 'lucy' [22]. MIRA assembler (v 3.2.0) was used to build an assembly of the V.test genome with the following flags and options: "-job = denovo, genome, accurate, sanger, 454 -highlyrepetitive -AS:klrs = no 454_SETTINGS -AS:urdcm = 1.1:ardml = 100''. This assembly yielded a very large contig containing a complete prDNA unit, and a second contig containing an incomplete unit bearing the prDNA/prDNA junction. The complete prDNA unit was extracted from the first contig and identified as being the last prDNA unit before the LUR junction and noted prDNA-G following Bublot et al. [14]. By analysing the contig bearing the prDNA/prDNA junction in GAP4 [23], we determined a 518 bp fragment of the prDNA-inner unit (as named by Bublot et al. [14]) bordered on the left by lower read qualities and coverage, and on the right by the beginning of a new prDNA unit. This end was joined to the beginning (2,089 bp) of the prDNA-G unit in order to obtain a complete prDNA-inner unit (2,607 bp). We verified that this complete unit was compatible with previously published information [14].

BoHV-4 genome annotation

All Open Reading Frames (ORFs) from all 6 frames were retrieved from the complete genomic sequence and matched against the Conserved Domain Database [24] using the position-specific scoring matrices (PSSM) based Reverse PSI-BLAST [25]. For all ORFs sharing the same STOP and containing a PSSM match, the smallest ORF containing the largest PSSM match was retained. 59 ORFs were thus considered evolutionarily conserved and were annotated with the corresponding matching conserved domains. Out of the 79 CDS from the previously published 66-p-347 strain, all 59 ORFs matched previously annotated 66-p-347 ORFs. The 20 remaining CDS were added by similarity to this strain and were annotated as such. Repeat segments and special features were annotated according to 66-p-347 if they were present in V.test. The complete genome sequence containing the LUR, prDNA-G and prDNA-inner were annotated and submitted to GenBank with respective accession numbers: JN133502, JN133503 and JN133504.

Comparative genomics analysis of 66-p-347 and V.test

The LUR and prDNA sequences of the 66-p-347 strain were joined into a complete genome (accession numbers NC_002665 and AF092919) and aligned against the joined LUR and prDNA-inner V.test sequences with ClustalW 2.0.10 [26]. Percent divergence, percent insertions and deletions, and percent G+C content were computed (i) along the alignment on a 100 bp sliding window of step 3 bp and (ii) on all individually aligned proteins. Analyses and figures were conducted using R [27] and the seqinr [28] package in combination with ad hoc programs written in Python and using the Biopython libraries [29, 30].

RT-PCR analysis

These experiments were performed as described elsewhere [31]. Briefly, subconfluent monolayers of MDBK cells were infected with BoHV4 V.test strain at a m.o.i. of 1 PFU/cell. 18 hours after infection, cytoplasmic RNA was extracted, purified and treated for RT-PCR. The cDNA products were amplified by PCR using specific primers listed in Table 1.

Results and discussion

BAC sequencing and genome assembly

Pyrosequencing of herpesviral genomes is often limited by the high concentration of contaminating cellular DNA [32]. We therefore prepared the BoHV-4 V.test strain DNA from BAC maintained genomes and sequenced it using a high-throughput pyrosequencing approach [16]. This yielded 48,967 reads among which 47,800 were BoHV-4 specific (> 97% of the reads). After assembly, the mean genome coverage was of the order of 96×. In comparison to the whole genome sequencing of another herpesvirus based on DNA isolated from virus particles, which exhibited a 13× average base pair coverage [32], our strategy showed a more than 7-fold increase. This is probably mainly due to the high proportion of viral to cellular reads present in our dataset. Indeed, only 1,167 Escherichia coli contaminant reads had to be discarded from the data, indicating less than 2.38% of contaminated reads, compared to the previously reported 62.72% contaminating cellular reads in [32]. Our sequencing strategy based on a BAC cloning approach, thus revealed itself very powerful in terms of contamination and subsequent coverage.

V.test genome analysis and comparison to other BoHV-4 strains

The BoHV-4 genome has a B-type structure consisting of a long unique region (LUR) flanked by several polyrepetitive DNA units (prDNA). We assembled the complete LUR of the V.test strain BoHV-4 genome into a 108,241 bp sequence. The average G+C content is of 41.21%. This value as well as the G+C% variation observed on Figure 1 is in agreement with previously reported results on the 66-p-347 strain, namely on the high G+C content of R2a region [11]. The observed-to-expected CpG ratio is of 0.225 on the LUR and is compatible with the value measured on Bos taurus (0.234) [33] suggesting (i) a high degree of methylation of CpG nucleotides and (ii) similar methylation mechanisms acting on the viral and cellular genome.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig1_HTML.jpg
Figure 1

Map of the BoHV-4 V.test strain genome and divergence with the 66-p-347 strain sequence. The LUR of both strains have been aligned. Genome features are represented in the upper part as grey and red oriented arrows. Red arrows represent genes with an in-frame STOP codon, an early Methionine or a high divergence level in the V.test strain compared to 66-p-347. Dark (resp. light) grey arrows represent genes with (resp. without) an evolutionarily conserved domain (see Methods). The exons of spliced genes are indicated under the given gene as thin light-blue lines. Percent divergence is shown as a black-filled curve, percent insertions and deletions are shown as a blue-filled curve. Percent G+C content is shown as a thin green curve, with the mean G+C content drawn as a thin horizontal green line. These percentages are measured in a 100 bp window sliding 3 bp. Repeat regions (R1, R2a, R2b) are depicted as hatched areas. The oriLyt region is mapped as a light-grey area within R2b and the conserved quasi-palindromic motif in the oriLyt region is indicated by a small vertical arrow.

As expected, the nucleotide identity between our assembled genome and previously published V.test strain sequence data was of 99.55% in average, falling into the ranges of comparison between 454 and Sanger sequencing [34].

Compared to the 66-p-347 strain, the V.test strain had previously shown divergence up to 12% on the region surrounding BORFB2 (ORF 16, v-Bcl-2) [11]. However, the lack of a complete genomic sequence for the V.test strain prevented from drawing a general conclusion concerning this divergence level. Compared to 66-p-347 strain, the overall V.test nucleotide identity is high (99.1%), but shows a large variability at the genome level (Figure 1). As expected, the repetitive regions contained in the LUR (R1, R2a and R2b) exhibit a high nucleotide divergence, up to more than 40%, as well as large gaps (Figure 1). This indicates that the very high divergence levels seem confined to specific repetitive genomic regions. However, some rather high divergence levels were also identified in other regions (Figure 1) and namely in ORF-containing regions such as ORF 10, Bo5, ORF 57, and ORF 68 region. We also note a large deletion and a high divergence at the beginning of the LUR compared to the 66-p-347 strain. Overall, these differences in protein-coding region as well as in repetitive regions that bear predicted microRNA coding sequences [35] will require specific experiments to identify possible links with observed phenotypic differences between strains.

Conserved protein-coding genes

In order to develop an ab initio approach of gene annotation, we extracted all possible ORFs in all 6 frames from the complete genomic sequence of the BoHV-4 V.test strain. On each of these ORFs, we ran a Reverse PSI-BLAST [25] against all protein domains from the Conserved Domain Database [24]. ORFs containing an evolutionarily conserved domain were defined as the smallest ORF containing the longest CDD match (see Methods). This approach revealed 59 ORFs containing a conserved CDD domain (Table 2). All 59 detected ORFs corresponded to ORFs previously annotated in the 66-p-347 strain (on a total of 79 ORFs listed in Table 3), indicating that 75% of BoHV-4 ORFs contain conserved domains. Most of these ORFs (37/59) contain domains that are either conserved at different levels in the Herpesvirales (either gammaherpesvirinae, herpesviridae or herpesvirales), or at a much larger scale that include Eukaryota, Bacteria and Archaea (22/59) (Table 2). This second set of genes might bear good candidates for genes having been the stage of lateral gene transfer events as observed for several herpesvirus genes [36] such as the BoHV-4 Bo17 gene that encodes a homologue of the cellular core 2 beta -1,6-N-acetylglucosaminyl-transferase M [37]. These results will deserve further studies to identify the evolutionary history responsible for these observations.
Table 2

Potential BoHV-4 V.test ORFs presenting conserved functional domains

BoHV-4 ORF

Domain accession number

Functional annotation

Species distributiona

ORF 3

COG0046, TIGR01735, TIGR01736, TIGR01739, PF02769

Phosphoribosylformylglycinamidine (FGAM) synthase; AIR synthase related protein

Eukaryota; Bacteria; Archaea; Viruses

ORF 6

PF00747

ssDNA binding protein

Herpesviridae

ORF 7

PF01366

Protein transporter activity

Herpesviridae

ORF 8

PF00606

Surface glycoprotein

Herpesviridae

ORF 9

COG0417, TIGR00592, PF00136, PF03104, SM00486

DNA polymerase type-B family

Eukaryota; Bacteria; Archaea; Viruses

ORF 10

PF04797

dUTPase

Herpesviridae

Bo4

COG5183, SM00744

Protein involved in mRNA turnover and stability; zinc-finger RING-variant domain

Eukaryota; Bacteria

ORF 16

SM00337

BCL (B-Cell lymphoma); contains BH1, BH2 regions

Eukaryota; Bacteria; Viruses

ORF 17

PF00716

Serine-type endopeptidase activity

Herpesviridae

ORF 18

PF03049

UL79 family

Herpesviridae

ORF 19

PF01499

Virus penetration and capsid assembly

Herpesviridae

ORF 20

PF01646

UL24 family

Herpesviridae

ORF 21

PF00693, PF01712, PF08465

ATP binding; thymidine kinase activity; phosphotransferase activity

Eukaryota; Bacteria; Viruses

ORF 22

PF02489

Virion associated envelope glycoprotein

Herpesviridae

ORF 23

PF04682

BTRF1 protein conserved region

Herpesviridae

ORF 24

PF03043

UL87 family

Herpesviridae

ORF 25

PF03122

Structural molecule activity

Herpesvirales

ORF 26

PF01802

Structural molecule activity

Herpesvirales

ORF 29

PF02499, PF02500

Probable role in DNA packaging

Herpesvirales

ORF 30

PF05338

Unknown function (DUF717)

Gammaherpesvirinae

ORF 31

TIGR01234, PF03048

UL92 family; L-ribulokinase

Herpesvirales; Embryophyta

ORF 32

PF04559

DNA cleavage and packaging

Herpesviridae

ORF 33

PF03044

Possible role in capsid maturation

Herpesviridae

ORF 34

PF03038

UL95 family

Herpesviridae

ORF 35

PF05852

Unknown function (DUF848)

Gammaherpesvirinae

ORF 36

COG0515, PF00069, SM00220

ATP binding; protein kinase activity; Serine/threonine protein kinase

Eukaryota; Bacteria; Archaea

ORF 37

PF01771, PF09588

Exonuclease activity; DNA binding

Eukaryota; Bacteria; Viruses

ORF 38

PF10813

Unknown function (DUF2733)

Herpesviridae

ORF 39

PF01528

Integral membrane protein

Herpesviridae

ORF 40

PF03324

Helicase-primase complex associated protein

Herpesviridae

ORF 41

PF05774

Helicase-primase complex components

Gammaherpesvirinae

ORF 42

PF01677

UL7-like protein

Herpesviridae

ORF 43

PF01763

Possible role in cleavage and packaging

Herpesviridae

ORF 44

COG0507, PF02689

Helicase; ATP-dependent exoDNAse (exonuclease V)

Eukaryota; Bacteria; Viruses

ORF 46

COG0692, TIGR00628, TIGR03443, PF03167

Uracil DNA glycosylase

Eukaryota; Bacteria; Archaea; Viruses

ORF 47

PF11108

Glycoprotein L

Herpesviridae

ORF 48

PF05734

Unknown function (DUF832)

Herpesviridae

ORF 50

PF03326, PF04793

Early-intermediate transcription factors

Gammaherpesvirinae

Bo10

PF05459, PF05812

Transcriptional regulator proteins

Herpesviridae

ORF 53

PF03554

Highly polymorphic glycoprotein

Herpesviridae

ORF 54

COG0756, TIGR00576, PF00692

dUTPase

Eukaryota; Bacteria; Archaea; Viruses

ORF 55

PF04533

U44 protein

Herpesviridae

ORF 56

PF03121

UL52/UL70 DNA primase

Eukaryota; dsDNA Viruses

ORF 57

PF04633

BMRF2 protein

Gammaherpesvirinae

ORF 59

PF04929

DNA replication accessory factor

Gammaherpesvirinae

ORF 60

COG0208, PF00268

Ribonucleotide reductase

Eukaryota; Bacteria; Archaea; Viruses

ORF 61

COG0209, TIGR02504, TIGR02506, TIGR02510, PF00317, PF02867

Ribonucleotide reductase

Eukaryota; Bacteria; Archaea; Viruses

ORF 62

PF03327

Capsid shell protein VP19C

Herpesviridae

ORF 63

PF04523

Tegument protein U30

Herpesviridae

ORF 64

PF04843

Tegument protein, N-terminal conserved region

Herpesviridae

ORF 65

PF06112

Capsid protein

Gammaherpesvirinae

ORF 66

PF03117

UL49 family

Herpesviridae

ORF 67

PF04541

Virion protein U34

Herpesviridae

ORF 67.5

PF03581

UL33-like protein

Herpesviridae

ORF 68

PF01673

Putative major envelope glycoprotein

Herpesviridae

ORF 69

PF02718

UL31-like protein

Herpesviridae

ORF 71

PF01335, SM00031

Death effector domain

Eukaryota; Viruses

ORF 75

COG0046, COG0047, TIGR01735, TIGR01736, TIGR01739, TIGR01857, PF02769

Phosphoribosylformylglycinamidine (FGAM) synthase; AIR synthase related protein

Eukaryota; Bacteria; Archaea; Viruses

Bo17

PF02485

Core-2/I-Branching enzyme

Eukaryota; Bacteria; Viruses

Conserved functional domains were determined for each BoHV-4 V.test ORF using METHOD and the domains accession numbers, functional annotation and the species distribution were listed. When several domains were conserved, the annotations were either merged when possible or juxtaposed. Domains present in herpesviridae conserved gene families are highlighted in bold (from Fu, 2008). (PFxxx: Pfam Accession Number; SMxxx: SMART Accession Number; COGxxx: COG Accession Number; TIGRxxx: TIGRFam Accession Number).

a Major taxonomic groups.

Table 3

Potential BoHV-4 V.test ORFs and homologues to HHV-8 and HHV-1

BoHV-4 ORF

Strand

Starta

Stopa

HHV-8 homologueb

HHV-1 homologuec

Annotation and comments

Bo1

-

272

460

--

--

Early in-frame STOP codon

ORF 3

+

441

4307

--

--

BORFA1; v-FGAM-synthase

Bo2

+

4435

4638

--

--

 

Bo3

+

5072

5299

--

--

 

ORF 6

+

5703

9107

ORF 6

UL 29

single-stranded DNA-binding protein MDBP

ORF 7

+

9112

11190

ORF 7

UL 28

transport protein

ORF 8

+

11180

13804

ORF 8

UL 27

glycoprotein B

ORF 9

+

13944

16961

ORF 9

UL 30

DNA polymerase

ORF 10

+

17057

18337

ORF 10

--

BORFB1

Bo4

-

18604

19101

--

--

short ORF of immediate early transcript 1

Bo5

-

19170

20355

K5

--

long ORF of immediate early transcript 1

Bo6

+

20297

20590

--

--

Early in-frame STOP codon

Bo7

-

20670

20994

--

--

Disrupted frame

Bo8

+

21318

21521

--

--

overlapping with late 1.7 kb RNA

ORF 16

+

22967

23647

ORF 16

--

BORFB2; v-Bcl-2 protein

ORF 17

-

23710

25260

ORF 17

UL 26

capsid protein

ORF 18

+

25259

26077

ORF 18

--

 

ORF 19

-

26029

27705

ORF 19

UL 25

tegument protein

ORF 20

-

27407

28219

ORF 20

UL 24

 

ORF 21

+

28203

29540

ORF 21

UL 23

thymidine kinase

ORF 22

+

29551

31674

ORF 22

UL 22

glycoprotein H

ORF 23

-

31671

32873

ORF 23

--

 

ORF 24

-

32851

35109

ORF 24

--

 

ORF 25

+

35099

39220

ORF 25

UL 19

major capsid protein

ORF 26

+

39256

40170

ORF 26

UL 18

capsid protein

ORF 27

+

40184

40829

ORF 27

--

 

Bo9

+

40831

41130

--

--

 

ORF 29

-

41154

46328

ORF 29

UL 15

cleavage/packaging protein

ORF 30

+

42306

42548

ORF 30

--

 

ORF 31

+

42482

43123

ORF 31

--

 

ORF 32

+

43069

44439

ORF 32

UL 17

viral DNA cleavage/packaging protein

ORF 33

+

44432

45430

ORF 33

UL 16

 

ORF 34

+

46327

47313

ORF 34

--

 

ORF 35

+

47285

47758

ORF 35

--

 

ORF 36

+

47787

48950

ORF 36

UL 13

kinase

ORF 37

+

48958

50427

ORF 37

UL 12

alkaline exonuclease

ORF 38

+

50379

50585

ORF 38

--

 

ORF 39

-

50652

51761

ORF 39

UL 10

glycoprotein M

ORF 40

+

51877

53247

ORF 40

UL 8

helicase-primase complex component

ORF 41

+

53360

53881

ORF 41

--

helicase-primase complex component

ORF 42

-

53873

54748

ORF 42

UL 7

 

ORF 43

-

54525

56375

ORF 43

UL 6

capsid protein

ORF 44

+

56323

58701

ORF 44

UL 5

helicase

ORF 45

-

58805

59530

ORF 45

--

 

ORF 46

-

59530

60291

ORF 46

UL 2

uracil-DNA-glycosidase

ORF 47

-

60309

60731

ORF 47

--

glycoprotein L

ORF 48

-

60838

62382

ORF 48

--

 

ORF 50

+

62586

65179

ORF 50

--

immediate early transcript 2; R transactivator protein

ORF 49

-

62600

63499

ORF 49

--

 

Bo10

+

65696

66595

--

UL 54

glycoprotein gp80

ORF 52

-

66621

67007

ORF 52

--

 

ORF 53

-

67073

67345

ORF 53

--

 

ORF 54

+

67414

68262

ORF 54

UL 50

dUTPase

ORF 55

-

68321

68923

ORF 55

--

 

ORF 56

+

68887

71418

ORF 56

UL 52

DNA replication protein

ORF 57

+

71512

72870

ORF 57

--

possible post-transcriptional transactivator

ORF 58

-

73294

74346

ORF 58

--

 

ORF 59

-

74360

75535

ORF 59

--

DNA replication protein

ORF 60

-

75688

76605

ORF 60

UL 40

ribonucleotide reductase small subunit

ORF 61

-

76639

79020

ORF 61

UL 39

ribonucleotide reductase large subunit

ORF 62

-

79002

80021

ORF 62

UL 38

assembly/DNA maturation protein

ORF 63

+

79978

82797

ORF 63

--

tegument protein

ORF 64

+

82812

90488

ORF 64

UL 36

tegument protein

ORF 65

-

90504

90896

ORF 65

--

capsid protein

ORF 66

-

90893

92167

ORF 66

--

 

ORF 67

-

92107

92877

ORF 67

UL 34

tegument protein

ORF 67.5

-

92881

93144

ORF 67.5

UL 33

Disrupted (late) methionine

ORF 68

+

93155

94504

ORF 68

UL 32

probable glycoprotein

ORF 69

+

94512

95405

ORF 69

UL 31

 

Bo11

-

96158

96703

--

--

 

Bo12

+

97442

97684

--

--

Early in-frame STOP codon

ORF 71

-

98328

98876

K13/ORF 71

--

BORFE2; v-FLIP

Bo13

+

98838

98983

--

--

Disrupted frame

ORF 73

-

99022

99783

ORF 73

--

BORFE3, LANA homologous

ORF 75

-

100274

103606

ORF 75

--

Disrupted (late) methionine; tegument protein/v-FGAM-synthetase

Bo14

-

104273

104785

--

--

 

Bo15

-

105724

106038

--

--

 

Bo16

+

106225

106494

--

--

 

Bo17

+

106681

108003

--

--

viral beta-1,6-N-acetylglucosaminyltransferase

a Positions of the respective ORFs of BoHV-4 on the LUR sequence. These are given from the first nucleotide of the start codon ATG to the last nucleotide of the stop codon.

b Correspondance to HHV-8 genes is according to Zimmermann (2001).

c Correspondance to HHV-1 genes was based on the presence of conserved domain with a BoHV-4 gene. Genes containing evolutionary conserved domains are highlighted in bold italic. The genes conserved in Herpesviridae are highlighted in bold (from Fu, 2008).

Non-conserved protein-coding genes

The remaining 20 annotated ORFs were determined by similarity to the 66-p-347 strain, and correspond for most of them to ORFs unique to BoHV-4 as described previously (Table 3) [11]. Some of these ORFs, however, contain odd characteristics that needed to be investigated (Figure 2, Additional file 1 Figures S1-9). Indeed Bo1, Bo6, Bo7, Bo12 and Bo13 genes of the BoHV-4 V.test strain present in-frame STOP codons. Bo5 presents rather high divergency levels and large insertions/deletions (> 5% of its coding sequence as shown in Figure 2) compared to the genomic sequence of the 66-p-347 strain. Moreover, ORFs 36, 67.5 and 75, which bear an evolutionary conserved domain, present late methionines compared to the 66-p-347 annotation. Indeed, in ORF 36 (see Additional file 1 Figures S5), the smallest ORF containing an evolutionary conserved domain is slightly shorter than the one annotated in 66-p-347 and there is no evidence that the previously annotated methionine is the correct one. However, comparison with homologous genes in other rhadinoviruses suggests that the start codon proposed in the 66-p-347 annotated sequence is the most likely. In ORF 67.5 (see Additional file 1 Figures S6), there is a point substitution in the 66-p-347 annotated ATG leading to the identification of a subsequent ATG as the V.test methionine. Finally, ORF 75 presents a small phase-disrupting indel in its 5' end (see Additional file 1 Figures S9), leading to the absence of the 66-p-347 annotated methionine in the V.test strain. All these annotated genes requested therefore an investigation of their transcription in mRNA products.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig2_HTML.jpg
Figure 2

Proteins in V.test: divergence with the previously published 66-p-347 strain. Percentage divergence and percentages indels on the aligned amino-acid sequences are represented as, respectively, black and light-blue bars. The dotted black line represents a 5% threshold. Genes containing an evolutionarily conserved domain (see Methods) are represented on a light-grey background. Previously annotated genes presenting in the V.test strain an in-frame stop codon, a late Methionine or large divergence levels compared to the 66-p-347 strain are indicated in red.

As these sequence properties could be specific to the BAC clone of the BoHV-4 V.test strain, we investigated the transcription of these genes on MDBK cells infected with the BoHV-4 V.test WT strain as described in the methods. The primers used are described in Table 1 and highlighted in Additional file 1. For all couple of primers, cDNA from BoHV-4-infected MDBK cells gave rise to the expected PCR products (Figure 3). The absence of contaminant viral DNA in the mRNA preparations was confirmed by a lack of PCR product without reverse transcriptase. The size of the Bo5 RT-PCR product was also consistent with its known mRNA splicing (868 bp rather than 1140 bp). Moreover, the sequences of these RT-PCR products were in agreement with the BoHV-4 V.test sequence derived from our BAC cloned genome (data not shown). Therefore, we can conclude that all these coding sequences are transcribed during BoHV-4 infection of MDBK cells. However, further investigation is needed to determine the presence of proteins and ensure their accurate annotation.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig3_HTML.jpg
Figure 3

RT-PCR amplification of the coding regions of the genes Bo1, Bo5, Bo6, Bo7, ORF67.5, Bo12, Bo13 and ORF75 of the BoHV-4 V.test strain. Subconfluent monolayers of MDBK cells were infected with BoHV4 V.test strain at a m.o.i. of 1 PFU/cell. 18 hours after infection, cytoplasmic RNA was extracted, purified and treated for RT-PCR. The cDNA products were amplified by PCR using specific primers listed in Table 1.

BoHV-4 V.test replication origin

A large region containing the potential lytic replication origin (oriLyt) of the BoHV-4 66-p-347 strain was determined by Zimmermann et al [11]. Based on this information, we mapped this region on the V.test genome (Figure 1). This region contains Bo12, the R2b region and partially overlaps with Bo11. Compared to the 66-p-347 strain sequence, the corresponding region in the V.test genome is highly divergent (Figure 4). Although this region shows high divergence rates, we expected the replication origin to be conserved between the two BoHV-4 strains. Previous work on other herpesviruses has identified in oriLyt the presence of palindromic motifs essential for viral replication [3840]. When we compared the potential region containing oriLyt in the two strains, a single conserved palindromic region was observed (AATCCAGGCCCCTGATTGGTAGATTGCTGAAAGCCAATCAGGGGCCTGGATT, Figure 4). Interestingly, this region forms a perfect hairpin structure (Figure 4B) that resembles DNA structures formed at other herpesvirus origins [41, 42] and may therefore represent a common secondary structure used by all herpesvirus family members during the initiation of DNA replication. In the future, this structure will be tested as a candidate for an essential oriLyt replication motif.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig4_HTML.jpg
Figure 4

Prediction of BoHV-4 V.test OriLyt. A. Alignment of the V.test strain (above) and 66-p-347 strain (below) regions predicted to contain the OriLyt in the 66-p-347 strain. The differences observed in the alignment are highlighted in light grey. The predicted potential OriLyt is highlighted in dark grey. B. The predicted secondary structures of the top (+) and bottom (-) strands of the predicted BoHV-4 OriLyt sequence were analyzed using the Vienna RNA website program RNAfold with DNA parameters. The predicted free energy (ΔG) of each structure is given, as well as the positional entropy of each nucleotide.

BoHV-4 V.test polyrepetitive DNA

In the BAC clone, previous restriction profiles had determined a hypermolar prDNA band indicating that the BAC contained several prDNA units [12]. Therefore, the major pitfall in the assembly of the BoHV-4 V.test strain was the determination of the prDNA sequence. Indeed, (i) the higher per base coverage on this region due to repetition of prDNA units, (ii) the high GC content, along with (iii) the presence of several long repeats within the prDNA and (iv) the variability observed between prDNA units [14] made it extremely difficult to resolve and assemble with pyrosequencing data alone. Interestingly, it has been shown for several rhadinoviruses that the left junction between the prDNA and the LUR is the site of genome rearrangements and that sequences of the prDNA are found within the first base pairs of the LUR. These properties make this region very difficult to sequence [4347].

Therefore, we adopted a hybrid strategy consisting in adding some ABI-Sanger reads (with the primers described in Table 1) to guide the 454 assembly on the prDNA region (see Methods).

Bublot, et al. [14] described the different prDNA unit variants present in BoHV-4 V.test, and namely the differences between prDNA units. Firstly, the prDNA units vary according to the number of repetitions of a ~200 bp Pst-I bordered fragment. Secondly, the last prDNA before the prDNA/LUR junction (prDNA-G) displays a different ending than the inner prDNA units [14]. Our method allowed us to disentangle the repeats and to assemble a contig containing a whole prDNA unit (2,440 bp) along with the left prDNA-LUR junction. This prDNA unit, corresponding to prDNA-G following Bublot et al. [14], was extracted from the contig and annotated. A second contig from this hybrid assembly yielded the prDNA/prDNA junction. The presence of the prDNA/prDNA junction in our assembly confirmed the presence of at least two prDNA units in our BAC clone and allowed us to build a complete prDNA-inner unit (see Methods). The assembled prDNA-G and inner prDNA units have sizes of 2,440 bp and 2,607 bp respectively. Both these units are in agreement with their previously published restriction maps [14].

Specifically, we showed that, comparatively to the 66-p-347 strain, the V.test prDNA-inner unit presents several indels including two large indels in the repetitive PstI region (Figure 5). This PstI-rich repetitive region seems to be the one presenting the most variation as it also presents comparatively large differences between prDNA units within the same strain. Indeed, Bublot et al. [14] roughly determined the size of the V.test major prDNA-inner unit to be around 2,650 bp due to the presence of 4 repetitions of the two small PstI bordered fragments.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig5_HTML.jpg
Figure 5

The BoHV-4 inner prDNA units contain conserved cleavage/packaging signals. Alignment of the prDNA-inner units from V.test strain (above) and 66-p-347 strain (below). The differences observed in the alignment are highlighted in grey. The cleavage/packaging signals pac-1 and pac-2 are represented in boxes, and their composing C-rich, G-rich, GC-rich and T-rich units are indicated. PstI, EcoRI, SstII, BamHI restriction sites are represented in coloured font.

In the prDNA-G unit, we established that these two small PstI-bordered fragments make up a fragment of 186 bp and that these are indeed repeated 4 times (Figure 6). In the prDNA-inner unit, we determined that the last PstI-bordered fragment is actually a variation of the 186 bp fragment where the inner Pst-I site is slightly modified (Figure 6). Therefore, the rough 200 bp size discrepancy between the prDNA-G (2,440 bp) and the prDNA-inner units (2,607 bp) is due to the presence of a slightly modified repetition of the previous segment. These results are compatible with the restriction profiles presented in Bublot et al. [14] as detailed by the positions of several restriction sites on Figure 6.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-8-406/MediaObjects/12985_2011_Article_1526_Fig6_HTML.jpg
Figure 6

The prDNA-G unit does not present a complete pac-2 cleavage/packaging signal. Alignment of the prDNA units from V.test strain (prDNA-inner above and prDNA-G below). The differences observed in the alignment are highlighted in grey. The cleavage/packaging signals pac-1 and pac-2 are represented in boxes, and their composing C-rich, G-rich, GC-rich and T-rich units are indicated. PstI, EcoRI, SstII, BamHI restriction sites (here PstI) are represented in coloured font. One modified PstI restriction site in the prDNA-inner is also highlighted to indicate the divergence between the fragments composing both units.

In addition to the variations in the PstI-bordered repetitions, one of the major differences between the prDNA-inner units and the prDNA-G lies in their 5' end. Indeed, the prDNA-inner contains a conserved pac-2 cleavage/packaging signal in its right terminal region, which is not the case of prDNA-G (Figure 6). Both units however, possess a conserved pac-1 cleavage/packaging signal in their left terminal region. Interestingly, the pac-1 and pac-2 cleavage and packaging signals show a good conservation between 66-p-347 and V.test's inner units, despite the presence of these signals in a repeated region bearing high divergence levels. Broll et al. [15] have determined, by transient cleavage/packaging assay, that a single prDNA unit is sufficient for cleavage and packaging. However, from the absence of a conserved pac-2 motif in the prDNA-G, we suggest that, even if a single inner prDNA unit is indeed sufficient for cleavage and packaging, the prDNA-G alone would not suffice. This would therefore indicate that two prDNA units at least are necessary in the context of naturally occurring BoHV-4 genomes for correct cleavage and packaging. The packaging of herpesvirus genomes is still not fully understood, however, detailed studies in herpes simplex virus type 1 (HSV-1), human and murine cytomegaloviruses (HCMV and MCMV) have highlighted the roles of the major conserved motifs and suggested the following general mechanism by which concatemers are cleaved and packaged [4850]. Firstly, the T-box of the pac-2 signal is essential for the cleavage that initiates DNA packaging. Cleavage occurs at a fixed distance from the pac-2 T-box, and the resulting end that contains the pac-2 GC-box and other cis acting elements is inserted into the procapsid. Packaging is therefore directional and proceeds from pac-2 towards the pac-1 terminus [48]. A second cleavage event, directed by pac-1, then terminates DNA packaging. If we apply this model to BoHV-4, the divergence of the pac-2 signal in prDNA-G, namely the absence of a T-box, indicates that it is not a functional pac-2 initiation signal. As the genome packaging is directional from pac-2 to pac-1 (therefore, from the right to the left end of the genome), the lack of a pac-2 initiation signal in prDNA-G ensures that no packaging would lead to a remaining concatemer lacking a left end prDNA. This would therefore guarantee that genomes bearing at least one left and one right end prDNA unit are encapsulated into virions. This model and its implications will require further investigations in the future.

Conclusions

BAC-cloning of the BoHV-4 V.test strain has greatly facilitated the use of this virus as a model for human pathogenic gammaherpesviruses. However, until now, the complete genome sequence of this strain was unavailable. In this study, we have determined the complete genome sequence of the BoHV-4 V.test strain. In comparison with the previously sequenced 66-p-347 strain, we identified important differences in 9 potential open reading frames. Moreover, sequence analyses allowed us to identify genome features that are potentially important for viral replication. All together, these results should have implications for the study of BoHV-4 and herpesviruses in general.

Declarations

Acknowledgements

LP is supported by a post-doctoral fellowship from the University of Liège. BM, CL and LG are Research Fellows and Research Associate of the "Fonds de la Recherche Scientifique - Fonds National Belge de la Recherche Scientifique" (F.R.S. - FNRS), respectively. This work was supported by the following grants: starting grant (D-09/11) and GLYVIR ARC of the University of Liège and scientific impulse grant of the F.R.S. - FNRS n°F.4510.10.

Authors’ Affiliations

(1)
Immunology-Vaccinology (B43b), Department of Infectious and Parasitic Diseases (B43b), Faculty of Veterinary Medicine, University of Liège

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