RESUMO
Banana streak virus (BSV) is a plant dsDNA pararetrovirus (family Caulimoviridae, genus badnavirus). Although integration is not an essential step in the BSV replication cycle, the nuclear genome of banana (Musa sp.) contains BSV endogenous pararetrovirus sequences (BSV EPRVs). Some BSV EPRVs are infectious by reconstituting a functional viral genome. Recent studies revealed a large molecular diversity of episomal BSV viruses (i.e., nonintegrated) while others focused on BSV EPRV sequences only. In this study, the evolutionary history of badnavirus integration in banana was inferred from phylogenetic relationships between BSV and BSV EPRVs. The relative evolution rates and selective pressures (d(N)/d(S) ratio) were also compared between endogenous and episomal viral sequences. At least 27 recent independent integration events occurred after the divergence of three banana species, indicating that viral integration is a recent and frequent phenomenon. Relaxation of selective pressure on badnaviral sequences that experienced neutral evolution after integration in the plant genome was recorded. Additionally, a significant decrease (35%) in the EPRV evolution rate was observed compared to BSV, reflecting the difference in the evolution rate between episomal dsDNA viruses and plant genome. The comparison of our results with the evolution rate of the Musa genome and other reverse-transcribing viruses suggests that EPRVs play an active role in episomal BSV diversity and evolution.
Assuntos
Badnavirus/genética , Evolução Molecular , Musa/virologia , Integração Viral , Badnavirus/enzimologia , Análise por Conglomerados , Genoma de Planta , Interações Hospedeiro-Patógeno , Modelos Genéticos , Musa/genética , Filogenia , Doenças das Plantas/genética , Doenças das Plantas/virologia , Ribonuclease H/genética , Seleção Genética , Proteínas Virais/genéticaRESUMO
The discovery of endogenous pararetroviral sequences (EPRVs) has had a deep impact on the approaches needed for diagnosis, taxonomy, safe movement of germplasm and management of diseases caused by pararetroviruses. In this article, we illustrate this through the example of yam (Dioscorea spp.) badnaviruses. To enable progress, it is first necessary to clarify the taxonomical status of yam badnavirus sequences. Phylogeny and pairwise sequence comparison of 121 yam partial reverse transcriptase sequences provided strong support for the identification of 12 yam badnavirus species, of which ten have not been previously named. Virus prevalence data were obtained, and they support the presence of EPRVs in D. rotundata, but not in D. praehensilis, D. abyssinica, D. alata or D. trifida. Five yam badnavirus species characterised by a wide host range seem to be of African origin. Seven other yam badnavirus species with a limited host range are probably of Asian-Pacific origin. Recombination under natural circumstances appears to be rare. Average values of nucleotide intra-species genetic distances are comparable to data obtained for other RNA and DNA virus families. The dispersion scenarios proposed here, combined with the fact that host-switching events appear common for some yam badnaviruses, suggest that the risks linked to introduction via international plant material exchanges are high.
Assuntos
Badnavirus/classificação , Dioscorea/virologia , Ecossistema , Doenças das Plantas/virologia , África , América , Sudeste Asiático , Austrália , Badnavirus/enzimologia , Badnavirus/genética , Dioscorea/classificação , Variação Genética , Melanesia , Dados de Sequência Molecular , Filogenia , DNA Polimerase Dirigida por RNA/genética , Recombinação Genética , Proteínas Virais/genéticaRESUMO
Rice tungro bacilliform virus (RTBV) is a plant pararetrovirus and member of the badnavirus subgroup. Open reading frame (ORF) 3 encodes the viral capsid protein, protease (PR), and reverse transcriptase (RT). A DNA fragment of ORF 3 that contains PR and RT sequences was previously expressed in insect cells to produce the PR/RT polyprotein that was processed to yield p62 and p55. p62 and p55 share common N-terminal amino acid sequences and exhibit reverse transcriptase activity. Mass spectrometry was employed to determine the precise molecular weight of the p62 and p55 proteins and enabled determination of the C-termini for both proteins. ORFs encoding either p62 or p55 were constructed and expressed in insect cells using the baculoviruses 62R-BBac and 55R-BBac, respectively. The recombinant p62R and p55R proteins were purified separately and shown to have the same enzymatic activities as previously reported for the processed p62 and p55. The putative active site of the PR was mutated (mpr), and the resulting mpr/RT ORF was expressed in insect cells using the baculovirus mpr/RT-BBac. The mpr/RT polyprotein was not processed in insect cells, resulting in the accumulation of the approximately 87-kDa mpr/RT polyprotein. This study further extends the understanding of p62 and p55 and clarifies the role of the RTBV PR in processing of the RT.
