Evolution of naturally occurring 5'non-coding region variants of Hepatitis C virus in human populations of the South American region
© Moratorio et al; licensee BioMed Central Ltd. 2007
Received: 03 May 2007
Accepted: 02 August 2007
Published: 02 August 2007
Hepatitis C virus (HCV) has been the subject of intense research and clinical investigation as its major role in human disease has emerged. Previous and recent studies have suggested a diversification of type 1 HCV in the South American region. The degree of genetic variation among HCV strains circulating in Bolivia and Colombia is currently unknown. In order to get insight into these matters, we performed a phylogenetic analysis of HCV 5' non-coding region (5'NCR) sequences from strains isolated in Bolivia, Colombia and Uruguay, as well as available comparable sequences of HCV strains isolated in South America.
Phylogenetic tree analysis was performed using the neighbor-joining method under a matrix of genetic distances established under the Kimura-two parameter model. Signature pattern analysis, which identifies particular sites in nucleic acid alignments of variable sequences that are distinctly representative relative to a background set, was performed using the method of Korber & Myers, as implemented in the VESPA program. Prediction of RNA secondary structures was done by the method of Zuker & Turner, as implemented in the mfold program.
Phylogenetic tree analysis of HCV strains isolated in the South American region revealed the presence of a distinct genetic lineage inside genotype 1. Signature pattern analysis revealed that the presence of this lineage is consistent with the presence of a sequence signature in the 5'NCR of HCV strains isolated in South America. Comparisons of these results with the ones found for Europe or North America revealed that this sequence signature is characteristic of the South American region.
Phylogentic analysis revealed the presence of a sequence signature in the 5'NCR of type 1 HCV strains isolated in South America. This signature is frequent enough in type 1 HCV populations circulating South America to be detected in a phylogenetic tree analysis as a distinct type 1 sub-population. The coexistence of distinct type 1 HCV subpopulations is consistent with quasispecies dynamics, and suggests that multiple coexisting subpopulations may allow the virus to adapt to its human host populations.
Hepatitis C virus (HCV) has infected an estimated 170 million people worldwide and therefore creates a huge disease burden due to chronic, progressive liver disease . Infections with HCV have become a major cause of liver cancer and one of the most common indications for liver transplantation [2–4]. The virus has been classified in the family Flaviviridae, although it differs from other members of the family in many details of its genome organization .
HCV is an enveloped virus with an RNA genome of approximately 9400 bp in length. Most of the genome forms a single open reading frame (ORF) that encodes three structural (core, E1, E2) and seven non-structural (p7, NS2-NS5B) proteins. Short untranslated regions at each end of the genome (5'NCR and 3'NCR) are required for replication of the genome. This process also requires a cis-acting replication element in the coding sequence of NS5B recently described . Translation of the single ORF is dependent on an internal ribosomal entry site (IRES) in the 5'NCR, which interacts directly with the 40S ribosomal subunit during translation initiation .
Comparison of nucleotide sequences of variants recovered from different individuals and geographical regions has revealed the existence of six major genetic groups . Each of the six major genetic groups of HCV contains a series of more closely related sub-types.
Little is known about the earlier divergence of the six major genotypes of HCV, the origins of infection in humans and the underlying bases of the current geographical distribution of genotypes. Some genotypes, such as 1a, 1b or 3a have become widely distributed and now are responsible for the vast majority of infections in Western countries .
Genotype 1 is the most prevalent type in the Latin American region . Previous and recent studies on genetic variation of HCV revealed a diversification of type 1 HCV strains circulating in that region [8–12]. There is no knowledge about the degree of genetic variability of HCV strains circulating in Bolivia and Colombia. This study aimed to elucidate these matters by performing a phylogenetic analysis of 5'NCR sequences from type 1 HCV strains recently isolated in Bolivia, Colombia and Uruguay, as well as available comparable sequences of HCV strains isolated in other regions of South America. In order to compare the results found for the South American region with other regions of the world, the same approach was used to perform a phylogenetic analysis of HCV strains isolated in Europe and North America.
Phylogenetic tree analysis of HCV strains isolated in the South American region
All HCV strains included in this study are clustered according to their genotype. Inside the main cluster of type 1 strains, different genetic lineages can be observed. One main line represents sub-type 1b strains (Fig. 1A, upper part), another represents type 1a strains (Fig. 1A, middle). Interestingly, type 1 HCV strains isolated in Bolivia, Colombia and some of the Uruguayan strains do not clustered together with major type 1 sub-types (1a and 1b). Instead, they are assigned to a different genetic lineage together with strains [EMBL:DQ077818], [EMBL:AY376833] and [EMBL:DQ313454], recently reported by Gismondi et al.[8, 9] and Schijman et al. (EMBL database submissions) as a new type 1 genetic lineage circulating in Argentina (see Fig. 1A, middle, cluster in red).
To observe if similar results can be found in other geographic regions of the world, the same studies were carried out for strains isolated in North America and Europe. The results of these studies are shown in Figs. 1B and 1C, respectively.
As it can be seen in the figures, while three different clusters can be clearly identified in HCV type 1 strains isolated in South America, this is not observed for type 1 strains isolated in North America or Europe (compare Fig. 1A with Figs. 1B and 1C).
Signature pattern analysis of type 1 HCV strains isolated in South America
Frequencies of signature nucleotides identified in the 5'NCR of type 1 HCV strains isolated in South Americaa
Frequency of query nucleotides
Frequency of background nucleotides
Among query set:
Among background set 1:
Among background set 2:
Among background set 3:
Prediction of secondary structure of signature RNA sequences
As it can be seen in Fig. 4, the predicted secondary structure of domains II of background and signature consensus sequences give similar structures. Nevertheless, mutation A107 in the sequence signature might help to stabilize a buckle in the structure by base pairing with U75 (compare Figs. 4A and 4B).
In the case of IRES stem-loop III predicted secondary structure, similar structures have also been obtained for background and signature sequences (see Fig. 5). Nevertheless, mutations in stem-loop III does not seem to have a particular effect in loop III folding (compare Figs. 5A and 5B).
Phylogenetic tree analysis of the 5'NCR from HCV strains isolated in South America revealed that genotype 1 is the most predominant in that region, in agreement with previous results . There are no previous reports on the genetic variation of HCV circulating in Bolivia. All Bolivian strains enrolled in these studies have been clearly assigned to genotype 1. Although more studies will be needed in order to have a definitive picture on the degree of genetic heterogeneity of HCV strains circulating in Bolivia, the results of these studies suggests that genotype 1 might also be prevalent in that country (see Fig. 1A). In the case of Colombia, previous studies suggested the presence of genotype 1 and 3 . This is in agreement with the results found in the present study. Interestingly, the phylogenetic analysis revealed the presence of genotype 4 in Colombia for the first time (see Fig. 1A, bottom). This genotype is prevalent in the Middle East  and not particularly in the South American region, although genotype 4 has been also found in Argentina . More studies will be needed to address the epidemiological situation of this genotype in Colombia.
The phylogenetic analysis of HCV strains isolated in South America also revealed the presence of a new genetic lineage in HCV type 1 strains (Fig. 1A). These results are in agreement with previous ones obtained for type 1 HCV isolates circulating in Central and South America [8–12]. These previous data have suggested the presence of a distinct type 1 HCV sub-population in South America and a diversification of HCV in that region. In this study, we have analyzed more than 150 HCV strains isolated in South America. The results of this work revealed that the third type 1 sub-population observed in the phylogenetic tree analysis of the HCV strains isolated in South America is in fact due to the presence of a particular nucleotide signature sequence (Fig. 2 and Table 1). This sequence signature is frequent enough to be detected in a phylogenetic tree analysis as a distinct type 1 sub-population (see Fig. 1A). Nevertheless, when the same analysis is carried out in type 1 HCV strains isolated in Europe or North America, only two genetic lineages are observed which correspond to the major type 1 sub-types (see Fig. 1B and 1C).
Sequence signature pattern analysis has been useful for epidemiological linkage, to corroborate transmission link hypothesis or sequence relatedness studies [18–21]. The identification of a sequence signature in the 5'NCR of type 1 HCV strains isolated in South America may permit a more in-depth study on the molecular epidemiology of HCV in this region.
Nevertheless, more studies will be needed to determine the extent of distribution of this particular signature. BLAST studies, on the other hand, have shown that only type 1 HCV strains circulating in the South American region have 100% similarity to the nucleotide sequence signature found in that region.
HCV, as many other RNA viruses, replicates as complex mutant distributions termed quasispecies [22–25]. Quasispecies dynamics is characterized by continuous generation of variant viral genomes, competition among them, and selection of the fittest mutant distributions in any given environment . The coexistence of distinct type 1 HCV subpopulations is consistent with quasispecies dynamics, and suggests that multiple coexisting subpopulations may occupy different regions on a fitness landscape to allow the virus to adapt rapidly to changes in the landscape topology. This, in turn, may allow the virus to adapt to its human host populations.
The 5'NCR, even though is one of the most conserved part of the virus genome, shows a quasispecies distribution with minor variants observed in the population  (Fig. 3). Since virus particles in serum are likely to be released from the liver but also from compartments such as lymphocytes or dendritic cells, it has been suggested that the sequence diversity found in the IRESs may reflect their translational activity and tropism for these compartments [27–29].
If all this is correct, the results of these studies may also be related to these facts. Owing to the error-prone nature of the HCV polymerase, mutations are expected to occur randomly distributed over the 5'NCR. However, only mutations compatible with replication and translation can be propagated. Whether the stem-loop II and III mutations observed confer a survival advantage or disadvantage in vivo remains unknown. Nevertheless, the in silico predicted RNA secondary structures of IRES stem-loops suggest that some mutations in the signature sequence might have an effect in IRES structure. Further work with HCV replicons containing the observed signature mutations may help to clarify this point.
The unique structure of the HCV IRES makes it an attractive target for the development of antiviral agents directed against this RNA element . Mapping sequence signatures in that region may help to understand their effects in HCV IRES functions.
Phylogenetic analysis revealed the presence of a sequence signature in the 5'NCR of type 1 HCV strains isolated in South America. This signature is frequent enough in type 1 HCV populations circulating South America to be detected in a phylogenetic tree analysis as a distinct type 1 sub-population. The coexistence of distinct type 1 HCV subpopulations is consistent with quasispecies dynamics, and suggests that multiple coexisting subpopulations may allow the virus to adapt to its human host populations.
Origins of Bolivian, Colombian and Uruguayan HCV strains
PCR amplification of 5'NCR of HCV strains
The 5'NCR of the HCV genome from samples that were reactive in the enzyme immunoassay were amplified by PCR, as previously described [31, 32]. To avoid false positive results, the recommendations of Kwok and Higuchi  were strictly adhered to. Amplicons were purified using QIAquick PCR Purification Kit from QIAGEN, according to instructions from the manufacturers.
Sequencing of PCR amplicons
The same primers used for amplification were used for sequencing the PCR fragments, and the sequence reaction was carried out using the Big Dye DNA sequencing kit (Perkin-Elmer) on a 373 DNA sequencer apparatus (Perkin-Elmer). Both strands of the PCR product were sequenced in order to avoid discrepancies. 5'NCR sequences from position 62 through 285 (relative to the genome of strain AF009606, sub-type 1A) were obtained. For sequence accession numbers of Bolivian, Colombian and Uruguayan HCV strains, see Table 2.
Phylogenetic tree analysis
5'NCR from HCV strains previously reported in South America, Europe and North America were obtained from the LANL HCV Database . Sequences were aligned using the CLUSTAL W program . Phylogenetic trees were generated by the neighbor-joining method under a matrix of genetic distances established under the Kimura-two parameter model , using the MEGA3 program . The robustness of each node was assessed by bootstrap resampling (1,000 pseudo-replicas).
Signature pattern analysis
HCV strains included in query and background datasets for sequence signature studiesa
[EMBL:AM266927], URU1, URU2, URU4, URU6, URU8, URU9, URU14, [EMBL:AM269928], [EMBL:AM269929], [EMBL:AM269930], [EMBL:AM269931], [EMBL:AM269932], [EMBL:AM269933], [EMBL:AM269934], [EMBL:AM269935], [EMBL:AM269936], [EMBL:DQ077818], [EMBL:DQ313454].
URUG7B, [EMBL:M84855], [EMBL:M84856], URU11, [EMBL:AB154179], [EMBL:AY576553], [EMBL:AY576557], [EMBL:DQ319979], [EMBL:M84838], [EMBL:M84839], [EMBL:M84841], [EMBL:AF077232], [EMBL:AF077236], [EMBL:AJ291457], [EMBL:AJ438617], [EMBL:AJ438619], [EMBL:AF011751], [EMBL:DQ010313], [EMBL:L34386].
[EMBL:AY576557], [EMBL:AY576576], [EMBL:DQ319979], [EMBL:DQ313980], [EMBL:DQ319983], [EMBL:M84838], [EMBL:M84840], [EMBL:M84841], [EMBL:M84842], [EMBL:Z84279], [EMBL:Z84280], [EMBL:D31722], [EMBL:AB154177], [EMBL:AB154178], [EMBL:Z84284], [EMBL:AB154179], [EMBL:AB154180], [EMBL:D31723], [EMBL:D31724].
[EMBL:AF009606], [EMBL:AY446036], [EMBL:AY446039], [EMBL:AY446043], [EMBL:AY446044], [EMBL:AY446049], [EMBL:AY446050], [EMBL:AY446051], [EMBL:AY446052], [EMBL:AY446053], [EMBL:AY446067], [EMBL:AY446068], [EMBL:DQ061296], [EMBL:DQ061297], [EMBL:DQ061299], [EMBL:L34377], [EMBL:L34385], [EMBL:L34388], [EMBL:L34389].
Sequence similarity studies
Prediction of RNA secondary structure
Secondary structure prediction was done by the method of Zuker & Turner , as implemented in the mfold program (version 3.2) . The core algorithm of this method predicts a minimum free energy, ΔG, as well as minimum free energies for foldings that must contain any particular base pair. The folding temperature was set to 37°C. Ionic conditions was set to 1M NaCl, non divalent ions. Base pairs that occur in all predicted folding structures are colored black. Otherwise, base pairs are assigned in a multi-color mode that displays precisely what foldings contain that base pair.
This work was supported by ICGEB, PAHO, and RELAB through Project CRP.LA/URU03-032, and DINACYT, Uruguay, through Project No. 8006. We thank Dr. Martín Abril, from Banco de Sangre de la Cruz Roja, Colombia for invaluable help in HCV samples collection.
We thank Gustavo Saez (Grupo CentraLab, Argentina) for RT-PCR related work with Argentinean HCV isolates.
- Simmonds P, Bukh J, Combet C, Deleage G, Enomoto N, Feinstone S, Halfon P, Inchauspe G, Kuiken C, Maertens G, Mizokami M, Murphy DG, Okamoto H, Pawlotsky JM, Penin F, Sablon E, Shin-IT , Stuyver LJ, Thiel HJ, Viazov S, Weiner AD, Widell A: Consensus proposals for a unified system of nomenclature of hepatitis C virus genotypes. Hematology 2005, 42: 962-973.Google Scholar
- Simmonds P: Genetic diversity and evolution of hepatitis C virus 15 years on. J Gen Virol 2004, 85: 3173-3188. 10.1099/vir.0.80401-0PubMedView ArticleGoogle Scholar
- Hoofnagle JH: Course and outcome of hepatitis C. Hepatology 2002, 36: S21-S29. 10.1002/hep.1840360704PubMedView ArticleGoogle Scholar
- Pawlotski JM: The nature of interferon-alfa resistance in hepatitis C virus infection. Curr Opin Infect Dis 2003, 16: 587-592. 10.1097/00001432-200312000-00012View ArticleGoogle Scholar
- You S, Stump DD, Branch AD, Rice CM: A cis -acting replication element in the sequence encoding the NS5B RNA-dependent RNA polymerase is required for hepatitis C virus RNA replication. J Virol 2004, 78: 1352-1366. 10.1128/JVI.78.3.1352-1366.2004PubMedPubMed CentralView ArticleGoogle Scholar
- Pestova TV, Shatsky IN, Fletcher SP, Jackson RJ, Hellen CUT: A prokaryotic-like mode of cytoplasmic eukaryotic ribosome binging to the initiation codon during internal translation initiation of hepatitis C and classical swine fever virus RNAs. Genes Dev 1998, 12: 6783.View ArticleGoogle Scholar
- Cristina J: Genetic diversity and evolution of hepatitis C virus in the Latin American region. J Clin Virol 2005, 34: S1-S7. 10.1016/S1386-6532(05)00393-8PubMedView ArticleGoogle Scholar
- Gismondi MI, Becker PD, Valva P, Guzman CA, Preciado MV: Phylogenetic analysis of previously nontypeable Hepatitis C virus isolates from Argentina. J Cin Microbiol 2006, 44: 2229-2232. 10.1128/JCM.02569-05View ArticleGoogle Scholar
- Gismondi MI, Staendner LH, Grinstein S, Guzman CA, Preciado MV: Hepatitis C virus isolates from Argentina disclose a novel genotype 1-associated restriction pattern. J Clin Microbiol 2004, 42: 1298-12301. 10.1128/JCM.42.3.1298-1301.2004PubMedPubMed CentralView ArticleGoogle Scholar
- San Roman M, Lezama L, Rojas E, Colina R, Garcia L, Carlos A, Khan B, Cristina J: Analysis of genetic heterogeneity of hepatitis C viruses in Central America reveals a novel genetic lineage. Arch Virol 2002, 147: 2239-2246. 10.1007/s00705-002-0869-4PubMedView ArticleGoogle Scholar
- Vega I, Colina R, García L, Uriarte R, Mogdasy C, Cristina J: Diversification of hepatitis C viruses in South America reveals a novel genetic lineage. Arch Virol 2001, 146: 1623-1629. 10.1007/s007050170084PubMedView ArticleGoogle Scholar
- Colina R, Azambuja C, Uriarte R, Mogdasy C, Cristina J: Evidence of increasing diversification of hepatitis C viruses. J Gen Virol 1999, 80: 1377-1382.PubMedView ArticleGoogle Scholar
- Felsenstein J: Phylogeny interference package, version 3.5. Department of Genetics, University of Washington, Seattle, U.S.A; 1993.Google Scholar
- Kuiken C, Yusim K, Boykin L, Richardon R: The HCV Sequence Database. Bioinformatics 2005, 21: 379-384. 10.1093/bioinformatics/bth485PubMedView ArticleGoogle Scholar
- Rijnbrand RC, Lemon SM: Internal ribosome entry site-mediated translation in hepatitis C replication. Curr Top Microbiol Immunol 2000, 242: 85-116.PubMedGoogle Scholar
- van Leeuwen HC, Reusken CB, Roeten M, Dalebout TJ, Riezu-Boj JI, Ruiz J, Spaan WJ: Evolution of naturally occurring 5'non-translated region variants of hepatitis C virus genotype 1b in selectable replicons. J Gen Virol 2004, 85: 1859-1866. 10.1099/vir.0.79924-0PubMedView ArticleGoogle Scholar
- Yepes A, Alvarez C, Restrepo JC, Correa G, Zapata JC: [Viral genotypes in patients with hepatitis C virus infection in Medellin] [Article in Spanish]. Gastroenterol Hepatol 2002, 25: 334-335.PubMedView ArticleGoogle Scholar
- Biswas S, Sanyal A, Hemadri D, Tosh C, Mohapatra JK, Manoj R, Bandyopadhvav SK: Sequence analysis of non-structural 3A and 3C protein-coding regions of foot-and-mouth disease virus serotype Asia 1 field isolates from an endemic country. Vet Microbiol 2006, 116: 187-193. 10.1016/j.vetmic.2006.03.001PubMedView ArticleGoogle Scholar
- Pistello M, Del Santo B, Butto S, Bargagna M, Domenici R, Bendinelli M: Genetic and phylogenetic analyses of HIV-1 corroborate the transmission link hypothesis. J Clin Virol 2004, 30: 11-18. 10.1016/j.jcv.2003.08.008PubMedView ArticleGoogle Scholar
- Burke B, Derby NR, Kraft Z, Sauders CJ, Dai C, Llewellyn N, Zharkikh I, Voitech L, Zhu T, Srivastava IK, Barnett SW, Stamatatos L: Viral evolution in macaques coinfected with CCR5-and CXCR4-tropic SHIVs in the presence or absence of vaccine-elicited anti-CCR5 SHIV neutralizing antibodies. Virology 2006, 355: 138-151. 10.1016/j.virol.2006.07.026PubMedView ArticleGoogle Scholar
- Soares MA, De Oliveira T, Brindeiro RM, Diaz RS, Sabino EC, Brigido L, Pires IL, Morgado MG, Dantas MC, Barreira D, Teixeira PR, Cassol S, Tanuri A: A specific subtype C of human immunodeficiency virus type 1 circulates in Brazil. AIDS 2003, 17: 11-21. 10.1097/00002030-200301030-00004PubMedView ArticleGoogle Scholar
- Chambers TJ, Fan X, Droll DA, Hembrador E, Slater T, Nickells MW, Dustin LB, Dibisceglie AM: Quasispecies heterogeneity within the E1/E2 region as a pretreatment variable during pegylated interferon therapy of chronic hepatitis C virus infection. J Virol 2005, 79: 3071-3083. 10.1128/JVI.79.5.3071-3083.2005PubMedPubMed CentralView ArticleGoogle Scholar
- Domingo E: Antiviral strategy on the horizon. Virus Res 2005, 107: 115-116. 10.1016/j.virusres.2004.11.001View ArticleGoogle Scholar
- Feliu A, Gay E, Garcia-Retortillo M, Saiz JC, Foms X: Evolution of hepatitis C virus quasispecies immediately following liver transplantation. Liver Transpl 2004, 10: 1131-1139. 10.1002/lt.20206PubMedView ArticleGoogle Scholar
- Laskus T, Wilkinson J, Gallegos-Orozco JF, Radkowski M, Adair DM, Nowicki M, Operskalski E, Buskell Z, Seeff LB, Vargas H, Rakela J: Analysis of hepatitis C virus quasispecies transmission and evolution in patients infected through blood transfusion. Gastroenterology 2004, 127: 764-776. 10.1053/j.gastro.2004.06.005PubMedView ArticleGoogle Scholar
- Lu M, Kruppenbacher J, Roggendorf M: The importance of the quasispecies nature of hepatitis C virus (HCV) for the evolution of HCV populations in patients: study on a single source outbreak of HCV infection. Arch Virol 145: 2201-2210. 10.1007/s007050070050
- Laporte J, Bain C, Maurel P, Inchauspe G, Agut H, Cahour A: Differential distribution and internal translation efficiency of hepatitis C virus quasispecies present in dendritic and livel cells. Blood 2003, 101: 52-57. 10.1182/blood-2002-03-0818PubMedView ArticleGoogle Scholar
- Laporte J, Malet I, Andrieu T: Comparative analysis of translation efficiencies of hepatitis C virus 5' untranslated regions among intraindividual quasispecies present in chronic infection: opposite behaviours depending on cell type. J Virol 2000, 74: 10827-10833. 10.1128/JVI.74.22.10827-10833.2000PubMedPubMed CentralView ArticleGoogle Scholar
- Lerat H, Shimizu YK, Lemon SM: Cell type-specific enhancement of hepatitis C virus internal ribosome entry site-directed translation due to 5' nontranslated region substitutions selected during passage of virus in lymphoblastoid cells. J Virol 2000, 74: 7024-7031. 10.1128/JVI.74.15.7024-7031.2000PubMedPubMed CentralView ArticleGoogle Scholar
- Kurreck J: Antisense technologies. Improvement through novel chemical modifications. Eur J Biochem 2003, 270: 1628-1644. 10.1046/j.1432-1033.2003.03555.xPubMedView ArticleGoogle Scholar
- Chan SW, McOmish F, Holmes EC, Dow B, Peutherer JF, Follett E, Yap PL, Simmonds P: Analysis of a new hepatitis C virus type and its phylogenetic relationship to existing variants. J Gen Virol 1991, 73: 1131-1141.View ArticleGoogle Scholar
- Davidson F, Simmonds P, Ferguson JC, Jarvis LM, Dow BC, Follett EA, Seed CR, Krusius T, Lin C, Madyuesi GA: Survey of major genotypes and subtypes of hepatitis C virus using RFLP of sequences amplified from the 5' non-coding regions. J Gen Virol 1995, 76: 1197-1204.PubMedView ArticleGoogle Scholar
- Kwok S, Higuchi R: Avoiding false positives with PCR. Nature 1989, 339: 237-238. 10.1038/339237a0PubMedView ArticleGoogle Scholar
- Thompson JD, Higgins DG, Gibson TJ: CLUSTAL W: improvingthe sensitivity of progressive multiple sequence alignment throughsequence weighting, position-specific gap penalties and weight matrix choice. Nucleic Acid Res 1994, 22: 4673-4680. 10.1093/nar/22.22.4673PubMedPubMed CentralView ArticleGoogle Scholar
- Kumar S, Tamura K, Nei M: MEGA 3: Integrated software for Molecular Evolutionary Genetics Analysis and sequence alignment. Brief Bioinformatics 2004, 5: 150-163. 10.1093/bib/5.2.150PubMedView ArticleGoogle Scholar
- Korber B, Myers G: Signature pattern analysis: a method for assessing viral sequence relatedness. AIDS Res Hum Retroviruses 1992, 8: 1549-1560.PubMedView ArticleGoogle Scholar
- Mathews DH, Sabina J, Zuker M, Turner DH: Expanded sequence dependence of thermodynamic parameters improve prediction of RNA secondary structure. J Mol Biol 1999, 288: 911-940. 10.1006/jmbi.1999.2700PubMedView ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.