Regulatory mutants of simian virus 40: Constructed mutants with base substitutions at the origin of DNA replication

Mutants of simian virus 40 (SV40) with base substitutions at or near the origin of replication of the viral genome have been constructed by bisulfite mutagenesis at the BglI restriction site of SV40 DNA, followed by transfection of cells with the BglI-resistant (BglI^r) DNA so generated. Based on plaque morphology at different temperatures, the resulting BglI^r mutants could be classified into four-groups. Class I mutants (designated ar for "altered restriction") were indistinguishable from wild-type SV40; class II mutants (designated shp for "sharp plaque") produced small, sharp-edged plaques; class III mutants (designated sp for "small plaque") produced small plaques at 32 °C, 37 °C and 40 °C; and class IV mutants (designated cs for "cold sensitive") produced small plaques at 32 °C and wild-type plaques at 37 °C and 40 °C. That the altered plaque morphology of sp and cs mutants was related to mutation at the BglI restriction site was demonstrated by co-reversion to wild-type of the plaque phenotype and BglI sensitivity. The nucleotide sequence around the original BglI site was determined in the DNA from one mutant of each class. In each case a different base-pair substitution was found, at a site outside sequences coding for SV40 proteins. When rates of replication of mutant DNAs were measured during productive infection, ar mutant DNA was synthesized at a rate comparable to that of wild-type SV40 DNA, shp mutant DNA was made at a rate exceeding that of wild-type, sp mutant DNA was synthesized at a lower rate than that of wild type. and cs mutant DNA synthesis was reduced at 32 °C, but about the same as the wild-type rate at 40 °C. These patterns of mutant DNA synthesis were unaltered in cells co-infected with mutant and wild-type virus, i.e. the defects in DNA synthesis were not trans-complementable. We conclude that the defective mutants have single base-pair changes in a cis element that determines the rate of viral DNA replication, presumably within the origin signal itself.