Bamboo mosaic virus (BaMV) has a single-stranded positive-sense RNA genome. The secondary structure of the 3-terminal sequence of the minus-strand RNA has been predicted by MFOLD and confirmed by enzymatic structural probing to consist of a large, stable stem-loop and a small, unstable stem-loop. To identify the promoter for plus-strand RNA synthesis in this region, transcripts of 39, 77, and 173 nucleotides (Ba-39, Ba-77, and Ba-173, respectively) derived from the 3 terminus of the minus-strand RNA were examined by an in vitro RNA-dependent RNA polymerase assay for the ability to direct RNA synthesis. Ba-77 and Ba-39 appeared to direct the RNA synthesis efficiently, while Ba-173 failed. Ba-77/⌬5, with a deletion of the 3-terminal UUUUC sequence in Ba-77, directed the RNA synthesis only to 7% that of Ba-77. However, Ba-77/⌬16 and Ba-77/⌬31, with longer deletions but preserving the terminal UUUUC sequence of Ba-77, restored the template activity to about 60% that of the wild type. Moreover, mutations that changed the sequence in the stem of the large stem-loop interfered with the efficiency of RNA synthesis and RNA accumulation in vivo. The mutant with an internal deletion in the region between the terminal UUUUC sequence and the large stem-loop reduced the viral RNA accumulation in protoplasts, but mutants with insertions did not. Taken together, these results suggest that three cis-acting elements in the 3 end of the minus-strand RNA, namely, the terminal UUUUC sequence, the sequence in the large stem-loop, and the distance between these two regions, are involved in modulating the efficiency of BaMV plus-strand viral RNA synthesis.There are at least two major steps involved in the replication of positive-sense viral RNA: the first is the synthesis of the complementary strand of genomic RNA, which is followed by the generation of progeny RNAs using the newly synthesized minus-strand RNA templates (37). For these reactions to proceed faithfully, the 3Ј ends of both plus-and minus-strand RNAs must contain important elements, the sequences and/or structures needed to direct the replicase to initiate the RNA synthesis (7, 18). The 3Ј-terminal secondary structures of Alfalfa mosaic virus RNAs (12,13,33), Turnip crinkle virus RNAs (39), and Tomato bush stunt virus defective interfering RNA (8) and the 3Ј tRNA-like structures (TLS) of Brome mosaic virus (BMV), Tobacco mosaic virus, and Turnip yellow mosaic virus (TYMV) RNAs have been identified to be important for minus-strand RNA synthesis (5,6,17,26,36).Bamboo mosaic virus (BaMV) is a single-stranded positivesense RNA virus. Its 6,366-nucleotide (nt) genome [excluding the poly(A) tail] contains a 5Ј m 7 GpppG structure and a 3Ј poly(A) tail (24). Five open reading frames (ORF) encoding polypeptides of 155, 28, 13, 6, and 25 kDa could be identified in the genome. Besides the genomic RNA which encodes the 155-kDa protein comprising a capping enzyme domain (21, 22), a 5Ј triphosphatase and helicase-like domain (9,11,16), and an RNA-dependent RNA polymerase (RdRp) activi...