Diversity in the 3' untranslated region of the genome of tobamoviruses isolated from Nicotiana glauca<1b>
Diversity in the 3' untranslated region of the genome of tobamoviruses isolated from Nicotiana glauca<1b>.
JA DODDS and S. BODAGHI
Department of Plant Pathology and the Microbiology Graduate Program. University of Califomia, Riverside, CA 92521, USA.
Background and objectives
Tobacco mild green mosaic tobamovirus (TMGMV) is found commonly in southern Caifornia in tree tobacco (Nicotiana glauca), which is a perennial shrub . It is reported to have a genome organization typical of other tobamoviruses . The 3' untranslated region of the genome includes a tRNA-like region at the terminus upstream of which is a domain composed of three pseudoknots. We report here population variability among tobamoviruses of the TMGMV type in this region of the genome
Results and Conclusions
Isolates could be assigned into 2 groups based on the size of the dsRNA for the subgenomic RNA that contains the ORF for the capsid protein plus the sequence of the 3'UTR, which was either large (22 isolates) or small (32 isolates). The small type could not be distinguished from a type strain of TMGMV. cDNA clones from the small type that in included the CpORF and the 3'UTR were obtained and sequenced and were found to closely resemble the published sequence for TMGMV. Corresponding clones from each of five large type isolates contained an insertional repeat. In four isolate this repeat started at position 39 (counting from the 3'terminus) and ended at position 185 and had the effect of duplicating the domain composed of three pseudoknots. One isolate contained an additional 19 bases of the T-RNA like structure (from the amino acyl receptor domain) in the repeat. The coat protein gene was nearly identical for all isolates and the TMGMV type strain. The isolates designated as small or type yielded twice as much virus after purification than the isolates designated as large. The two types of isolates could be distinguished in N. rustica (large but not small gave a systemic infection), N. clevelandii (small but not large induced systemic lethal necrosis) and N. benthamiana (large but not small induced rapid wilting). Repeats in the 3' UTR have been previously reported for one other tobamovirus (odontogiossum ringspot virus), but no variants lacking the repeat are known. Satellite tobacco mosaic virus mimics tobamoviruses in the 3' UTR (but not the coat protein) and all isolates studied to date have repeats in the 3' UTR. All isolates of TMGMV used in this study were initially associated with STMV. What might be driving this population diversity in TMGMV in N. glauca is unclear, but the near identity of the sequence in the repeat, together with the differences in sizes of the repeat, suggest this may be an active process. It is interesting to speculate if this type of flexibility is a feature of natural populations of other tobamoviruses or other viruses with similar 3'UTR's.
1 Valverde, RA., Dodds, JA. 1986 J. Gen. virol. 67:1875-1884.
2. Solis I., Garcia-Arenal, F. 1990. Virology. 177. 553-558.