Regulation of single-strand RNA synthesis of alfalfa mosaic virus in non-transgenic cowpea protoplasts by the viral coat protein

Summary

We have compared the RNA synthesis of alfalfa mosaic virus in complete (by RNAs 1, 2 and 3) and incomplete infections (by RNAs 1 and 2) of cowpea protoplasts. Both viral RNA polymerase activity and accumulation of viral RNA were measured. By annealing RNA in solution with ³²P-labelled probes of plus and minus polarity followed by treatment with ribonucleases, we determined viral RNAs quantitatively in both single- and double-stranded RNA fractions. The accumulation of single-stranded RNA of positive polarity differed considerably between the two types of infection (250 ng vs. less than 1 ng per 10⁵ protoplasts), although viral RNA polymerase activities as measured in vitro and the concentrations of minus RNA were similar. Since the method also measured fragmented RNA, this difference is probably not due to lack of protection of viral RNA by coat protein during incomplete infection. Synthesis of single-stranded plus RNA requires either RNA 3 itself or one of its gene products. We postulate that coat protein is the stringent regulator of alfalfa mosaic virus genomic expression.