DIVERSITY AMONG ISOLATES OF SQUASH MOSAIC-VIRUS

cDNA clones of RNA-2 of two isolates of squash mosaic virus (SqMV) wer e constructed and sequenced, revealing 87% sequence similarity. In Nor thern blot hybridization analyses, DNA probes made from these clones d efined two SqMV hybridization subgroups. This grouping was verified by reciprocal hybridizations of purified RNA from five SqMV isolates, as probed with cDNA made from a member of each subgroup. Comparison of t he RNA-2 sequence among the two SqMV isolates, and the reported sequen ce of other comoviruses, showed that SqMV constitutes one of four majo r branches in a phylogenetic tree of the genus. Analysis of the termin al noncoding sequences showed that although potentially similar foldin g patterns may form, neither nucleotide sequence nor secondary structu ral elements are highly conserved among comoviruses. In vitro translat ion products from purified RNA-1 of each subgroup (encoding the viral proteases) were found to process the polyprotein generated by in vitro translation of purified RNA-2 from either subgroup.