Bioinformatic evidence for a stem-loop structure 5'-adjacent to the IGR-IRES and for an overlapping gene in the bee paralysis dicistroviruses
© Firth et al. 2009
Received: 23 September 2009
Accepted: 06 November 2009
Published: 06 November 2009
The family Dicistroviridae (order Picornavirales) includes species that infect insects and other arthropods. These viruses have a linear positive-sense ssRNA genome of ~8-10 kb, which contains two long ORFs. The 5' ORF encodes the nonstructural polyprotein while the 3' ORF encodes the structural polyprotein. The dicistroviruses are noteworthy for the intergenic Internal Ribosome Entry Site (IGR-IRES) that mediates efficient translation initation on the 3' ORF without the requirement for initiator Met-tRNA. Acute bee paralysis virus, Israel acute paralysis virus of bees and Kashmir bee virus form a distinct subgroup within the Dicistroviridae family. In this brief report, we describe the bioinformatic discovery of a new, apparently coding, ORF in these viruses. The ORF overlaps the 5' end of the structural polyprotein coding sequence in the +1 reading frame. We also identify a potential 14-18 bp RNA stem-loop structure 5'-adjacent to the IGR-IRES. We discuss potential translation initiation mechanisms for the novel ORF in the context of the IGR-IRES and 5'-adjacent stem-loop.
Dicistrovirus sequences were extracted from GenBank, the polyprotein coding sequences were extracted, translated, aligned with CLUSTALW , back-translated to nucleotide sequence alignments, and clustered into separate alignments for each GenBank dicistrovirus RefSeq (using 65% nucleotide identity to the RefSeq as a cut-off threshold). Beginning with pairwise sequence comparisons, conservation at synonymous sites (only) was evaluated by comparing the observed number of base substitutions with the number expected under a neutral evolution model. The procedure takes into account whether synonymous site codons are 1-, 2-, 3-, 4- or 6-fold degenerate and the differing probabilities of transitions and transversions (see  for details). Statistics were then summed over a phylogenetic tree as described in , and averaged over a sliding window.
Next, the bee paralysis virus CDS2 alignment was analysed with MLOGD - a gene-finding program which was designed specifically for identifying overlapping coding sequences, and which includes explicit models for sequence evolution in multiply-coding regions [13, 14] (Figure 2B, panels 8-11). Due to the overall high conservation, the absolute MLOGD scores tend to be low within the ORFX region (since there are fewer substitutions with which to discrimate the null or non-coding model from the alternative or coding model). Nonetheless, MLOGD predicts that ORFX is indeed a coding sequence, with consecutive positively-scoring windows in the ORFX region (Figure 2B, panels 9 and 11).
Given the location of ORFX and the unusual translation mechanism of CDS2, the translation of ORFX - if it is indeed expressed - is clearly of interest and may provide new insights into the mechanics of IGR-IRES mediated initiation. Possible ORFX translation mechanisms include (i) a portion of ribosomes initiate at more-or-less the normal IGR-IRES mediated non-Met-tRNAi initiation site but in the +1 frame; (ii) a portion of ribosomes, or rather 40S ribosome subunits, binding to the IGR-IRES somehow start scanning, and normal AUG-initiation takes place at a conserved tandem pair of +1 frame AUG codons ~35 codons downstream (Figure 3A) or, in some sequences, at AUG codons further 5'; or (iii) normal IGR-IRES mediated CDS2 initiation occurs but is followed by a programmed +1 frameshift into ORFX.
The synonymous site conservation plot peaks around the tandem +1 frame AUG codons (Figure 2B, panel 7; Figure 3A), falling off rapidly upstream and more slowly downstream. However, it is unclear whether or not this favours scanning and AUG initiation. There is still significant synonymous site conservation upstream of the AUG codons (Figure 3A). The peak in synonymous site conservation may just represent the region of the putative protein that is subject to the strongest amino acid constraints. The MLOGD statistics, on the other hand, indicate that the positive coding signature in the +1 frame extends right up to the 5' end of CDS2 (Figure 2B, panel 11), thus favouring the model in which a portion of ribosomes initiate at or near the usual IGR-IRES initiation site but in the +1 reading frame.
If ORFX initiation occurs at the normal IGR-IRES initiation site but in the +1 frame then translation of ORFX would result in an 11.2 kDa, 93 amino acid product in KBV, and 92 and 94 amino acid products in ABPV and IAPV respectively. If, however, initiation takes place at the downstream tandem AUG codons, then translation of ORFX would result in a 7.1 kDa, 60 amino acid product in all three species. Within the longer (i.e. 92-94 amino acid) potential ORFX product, there are 61 residues that are completely conserved between the KBV, ABPV and IAPV GenBank RefSeqs. In the region of the structural polyprotein that is encoded by the portion of the CDS2 sequence that ORFX overlaps, there are 66 completely conserved residues. Thus the putative ORFX product is apparently subject to slightly weaker functional constraints than the 'corresponding' portion of the structural polyprotein.
The IGR-IRESes of the bee paralysis viruses differ from the IGR-IRESes of most other sequenced dicistroviruses in one notable aspect - namely they have an extra hairpin structure within domain 3 (see Refs. [11, 20] for details). We investigated the possibility that the presence of the extra hairpin structure might be correlated with the presence of ORFX. Two other sequenced dicistroviruses have the extra hairpin structure - (i) the ant-infecting Solenopsis invicta virus 1 or SINV-1 ([GenBank:NC_006559]; [21, 22]), and (ii) the shrimp-infecting Taura syndrome virus or TSV ([GenBank:NC_003005]; ).
SINV-1 clusters with the bee paralysis viruses in the phylogenetic tree (Figure 1), and an analysis of its sequence shows that it does indeed contain a potential ORFX. In fact ORFX in SINV-1 is substantially longer than in the bee paralysis viruses - 125 codons if initiated in the +1 frame at the IGR-IRES normal initiation site; 83 codons if initiated at the tandem AUG codons (which are present in SINV-1 and align with the tandem AUG codons in the bee paralysis viruses); or 121 codons if initiated at an unstream intervening AUG codon (Figure 3A). (An additional SINV-1 sequence - [GenBank:FJ229495] - with partial coverage of the ORFX region contained an ORFX-frame premature termination codon [PTC] that truncates ORFX by 33 codons. However, apart from the potential for sequencing errors, PTCs in a small number of isolates are not unusual for short overlapping genes, which tend to have non-essential 'secondary' functions, and we do not believe that this ORFX-defective partial sequence necessarily precludes the presence of a functional ORFX in SINV-1.)
On the other hand, ORFX was not present in TSV. The first +1 frame AUG codon 3' of the IGR-IRES initiation site is preceded by a CDS2-frame AUG codon, and is closely followed by a +1 frame stop codon, while non-AUG +1 frame initiation at the usual IGR-IRES initiation site would only give a 16 amino acid product. Thus the presence of ORFX does not seem to correlate with the presence of the extra hairpin structure within domain 3 of the IGR-IRES.
However, we did identify a novel (so far as we are aware) potential RNA hairpin structure immediately 5'-adjacent to, but not overlapping, the IGR-IRES in the bee paralysis viruses (Figure 3C). In the KBV and IAPV RefSeqs, the hairpin comprises 18 consecutive base pairs (with a 4 nt terminal loop containing the CDS1 termination codon) and is supported by many compensatory substitutions (i.e. paired substitutions that maintain the base pairings) between KBV and IAPV. Inspection of 77 additional sequences with coverage of this region revealed six (mostly identical) sequences with single mismatches in the stem, one sequence with two mismatches, and one sequence with a 4-nt deletion at the apical end of the stem. Nonetheless, the majority of sequences retained a perfect 18 bp hairpin, and a total of 14 different substitutions that preserved the base pairings were observed. A similar, though shorter (14 bp), hairpin stucture was identified in ABPV (Figure 3C). Again, inspection of ten additional sequences revealed five different substitutions in the stem, all of which preserved the predicted base pairings. Whether the hairpin is in any way relevant to translation of the putative ORFX remains to be seen. However, preliminary experimental results indicate that presence of the predicted hairpin does have a strong effect on IGR-IRES activity (unpublished data, QS Wang and E Jan).
Recent results suggest that under certain circumstances (namely the presence of an initiator tRNA species that recognizes the P-site codon) the IGR-IRES can, at some level, mediate initiation at the P-site (presumably in competition with A-site initiation) . The codon:anticodon duplex mimicking part of the IGR-IRES (a.k.a. PKI) has been shown to be dynamic and flexible [25–27], and Ref.  suggest that P-site initation takes place only upon dissociation of the duplex. However, this duplex is critical for selection of the CDS2 reading frame  so, upon dissociation of the duplex, there may be flexibility in the selection of reading frame, thus perhaps allowing +1 frame P-site initiation. In fact, all available bee paralysis virus sequences have a CUG codon at this location, which is known to be recognizable by native Met-tRNAi .
Other dicistroviruses lack a long overlapping ORF at this genomic location and lack the corresponding extended region of synonymous site conservation (data not shown). At least some other dicistroviruses do exhibit some degree of heightened synonymous site conservation at the very 5' end of CDS2, but the 3' extent of these regions appears to be much more limited than in the bee paralysis viruses (perhaps it simply reflects selection against certain nucleotides in order to avoid forming alternative RNA secondary structures that may disrupt IGR-IRES activity ). In fact the sequence data is rather limited for most dicistroviruses in the sense that it is difficult to make alignments with sufficiently large phylogenetically-summed diversity but sufficiently small pairwise divergences for the above analyses to produce useful statistics. Thus, there may be features in the other dicistroviruses that will remain hidden until more sequence data becomes available.
Overlapping genes are difficult to identify and are often overlooked. However, it is important to be aware of such genes as early as possible in order to avoid confusion (otherwise functions of the overlapping gene may be wrongly ascribed to the gene they overlap), and also so that the functions of the overlapping gene may be investigated in their own right. Although overlapping the structural polyprotein, there is no reason to suspect that ORFX encodes a structural protein - indeed the limited phylogenetic distribution of ORFX suggests that it does not. We are currently investigating the translation mechanism for the putative ORFX and how it relates to the IGR-IRES and the potential upstream hairpin structure.
Note: during the preparation of this manuscript, the positive coding potential of ORFX was also predicted by Ref.  (who name the ORF 'pog'), albeit using different bioinformatic approaches.
This work was supported by National Institutes of Health Grant R01 GM079523 and an award from Science Foundation Ireland, both to JFA.
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