SPECIFIC CESSATION OF MINUS-STRAND RNA ACCUMULATION AT AN EARLY STAGE OF TOBACCO MOSAIC-VIRUS INFECTION

The time course of accumulation of viral plus-strand RNAs (genomic RNA and subgenomic mRNA for the coat protein) and minus-strand RNA in tobacco protoplasts synchronously infected with tobacco mosaic virus (TMV) RNA was examined. In protoplasts infected with the wild-type TMV L RNA, the plus and minus strands accumulated differently not only in quantity but also in the outline of kinetics. The time courses of accumulation of the genomic RNA and coat protein mRNA were similar: they became detectable at 2 or 4 h postinoculation (p.i.), and their accumulation increased until 14 to 18 h p.i. The accumulation rate reached the maximum at about 4 h p.i. and then gradually decreased. In contrast, accumulation of the minus-strand RNA ceased at 6 to 8 h p.i., at which time the plus-strand accumulation was already about 100 times greater and still continued vigorously. This specific halt of minus-strand accumulation was not caused exclusively by encapsidation of the genomic RNA, because a similar halt was observed upon infection with a deletion mutant that lacks the 30K and coat protein genes. Upon infection with a mutant that could not produce the 130K protein (one of the two proteins that are thought to be involved in viral RNA replication), the accumulation levels of both plus and minus strands were lower than that of the parental wild-type virus. Given these observations, possible mechanisms of TMV replication are discussed.