EFFECTS OF COAT PROTEIN MUTATIONS AND REDUCED MOVEMENT PROTEIN EXPRESSION ON INFECTION SPREAD BY COWPEA CHLOROTIC MOTTLE VIRUS AND ITS HYBRID DERIVATIVES

Previously we have reported that the essential 3a movement gene of ico sahedral cowpea chlorotic mottle virus (CCMV) can be functionally repl aced by the 30-kDa movement gene of rod-shaped sunn-hemp mosaic virus (SHMV). Because plant RNA viruses differ in requiring or not requiring coat protein for systemic infection, we have now investigated whether systemic spread by this CCMV/SHMV hybrid is dependent on its CCMV coa t protein as well as its SHMV movement protein. We find that either de letion or frameshift mutations in the coat protein gene block systemic spread. Thus, like wild-type CCMV, systemic infection by the hybrid i s dependent on both movement protein and coat protein. These results f urther support the conclusion that the required functions of the coat and movement proteins in CCMV spread do not depend on sequence-specifi c interaction between these proteins. Additional features of the hybri d also motivated testing the effects of modulating movement protein ex pression. Creating an extra, out-of-frame translational start codon (A UG) shortly upstream of the 3a movement protein gene in CCMV downregul ated its expression 18-fold. Nevertheless, for CCMV derivatives bearin g either the CCMV 3a gene or the SHMV 30-kDa gene, the extra AUG resul ted in only a minor delay in the onset of viral spread and little or n o effect on the subsequent rate of cell-to-cell spread. Thus, under no rmal circumstances, the rate of CCMV cell-to-cell spread in cowpea pla nts appears to be limited primarily by factors other than movement pro tein synthesis. (C) 1997 Academic Press.