YEAST POSITIVE-STRANDED VIRUS-LIKE RNA REPLICONS - 20-S AND 23-S RNA TERMINAL NUCLEOTIDE-SEQUENCES AND 3'-END SECONDARY STRUCTURES RESEMBLETHOSE OF RNA COLIPHAGES

Saccharomyces cerevisiae strains carry single-stranded RNAs called 20 S RNA and 23 S RNA. These RNAs and their double-stranded counterparts, W and T dsRNAs, have been cloned and sequenced, A few nucleotides at both ends, however, remained unknown, These RNAs do not encode coat pr oteins but their own RNA-dependent RNA polymerases that share a high d egree of conservation to each other. The polymerases are also similar to the replicases of RNA coliphages, such as Q beta, Here we have dete rmined the nucleotide sequences of W and T dsRNAs at both ends using r everse transcriptase polymerase chain reaction-generated cDNA clones, We confirmed the terminal sequences by primer-extension and RNase prot ection experiments. Furthermore, these analyses demonstrated that W an d T dsRNAs and their single-stranded RNA counterparts (i) are linear m olecules, (ii) have identical nucleotide sequences at their ends, and (iii) have no poly(A) tails at their 3' ends. Both 20 S and 23 S RNAs have GGGGC at the 5' ends and the complementary 5-nucleotides sequence , GCCCC-OH, at their 3' ends. S1 and V1 secondary structure-mapping of the 3' ends of 20 S and 23 S RNAs shows the presence of a stem-loop s tructure that partially overlaps with the conserved 3' end sequence. N ucleotide sequences and stem-loop structures similar to those describe d here have been found at the 3' ends of RNA coliphages, These data, t ogether with the similarity of the RNA-dependent RNA polymerases encod ed among these RNAs and RNA coliphages, suggest that 20 S and 23 S RNA s are plus-strand single-stranded virus-like RNA replicons in yeast.