Role of alfalfa mosaic virus coat protein in regulation of the balance between viral plus and minus strand RNA synthesis.

Replication of wild type RNA 3 of alfalfa mosaic virus (AIMV) and mutants with frameshifts in the P3 or coat protein (CP) genes was studied in protoplasts from tobacco plants transformed with DNA copies of AIMV RNAs 1 and 2. Accumulation of viral plus and minus strand RNAs was monitored with strand-specific probes. A frameshift in the P3 gene did not change the asymmetry in plus/minus strand accumulation observed for the wild type. A frameshift early in the CP gene resulted in a 100-fold reduction in plus strand accumulation and a 3- to 10-fold increase in minus strand accumulation. A frameshift late in the CP gene caused a similar reduction in plus strand accumulation but had no effect on minus strand accumulation. This latter mutant accumulated nearly wild type levels of a truncated CP molecule. Apparently, wild type AIMV CP up-regulates plus strand accumulation and down-regulates minus strand accumulation and these two functions can be mutated separately.