Poa semilatent virus, a hordeivirus having no internal polydisperse poly(A) in the 3' non-coding region of the RNA genome

Abstract

RNA from the Hungarian isolate of poa semilatent virus (PSLV) directed in vitro synthesis of 120K, 75K, 25K (coat protein) and 20K polypeptides. In vitro translation of PSLV RNA was blocked by the cap analogue, m7Gpp, thus suggesting that the virus RNA was capped. PSLV RNA could be aminoacylated with [14C]tyrosine in vitro. The sequence of 1.5 kb from the 3' end of the PSLV RNA gamma component revealed two open reading frames (ORFs) separated by a uridine-rich intergenic region. The putative product of the incomplete 5'-proximal ORF showed a close amino acid sequence similarity with the C-terminal segment of the gamma a protein (putative RNA replicase) encoded in the barley stripe mosaic virus (BSMV) RNA gamma, and the 20K product of the 3'-proximal ORF was found to be related to the 17K gamma b product of BSMV. The sequence of 0.8 kb from the 3' end of PSLV RNA beta encompassed two (incomplete) overlapping ORFs whose putative products are related to the beta c and beta d proteins encoded in the similarly arranged ORFs of BSMV RNA beta. Nucleotide sequence homology between the respective parts of the two hordeivirus genomes was restricted to the ORF for gamma a, the spacer between the ORFs for gamma a and gamma b, and the 3' non-coding region, particularly the 95 nucleotide segment at the 3' end representing a tRNA-like structure. Despite limited sequence conservation beyond this segment, the entire 3' non-coding region of PSLV RNA could be folded in a tight pseudoknotted structure closely resembling that of BSMV RNA. Surprisingly, the 'signature' sequence typical for BSMV RNA, internal polydisperse poly(A) intercalated between the coding part of the 3' tRNA-like structure, was not detected in the PSLV genome. Instead, the virus RNA contained several oligoadenylate stretches spaced by other residues, close to the junction of its coding and 3' non-coding portions.