Compensatory capsid protein mutations in cucumber mosaic virus confer systemic infectivity in squash (Cucurbita pepo)

Cucumber mosaic virus (CMV) systemically infects both tobacco and zucchini squash. CMV capsid protein loop mutants with single-amino-acid substitutions are unable to systemically infect squash, but they revert to a wild-type phenotype in the presence of an additional, specific single-site substitution. The D118A, T120A, D192A, and D197A loop mutants reverted to a wild-type phenotype but did so in combination with P56S, P77L, A162V, and I53F or T124I mutations, respectively. The possible effect of these compensatory mutations on other, nonsystemically infecting loop mutants was tested with the F117A mutant and found to be neutral, thus indicating a specificity to the observed changes.