The construction of the infectious clone pGGVs for Vs virus and methods of mutagenesis have been described (34, 36) . Briefly, the pGGVs was subcloned into two plasmids; one, pGGVs 660 contained the first 660 nt of the virus genome and the second pGGVs 660-10927 included the remainder. Site-directed mutagenesis was accomplished by PCR (details of primers are available on request). Mutated PCR products were cloned into the pGGVs 660 between MluI and EcoRI sites followed by sequencing.