A temperature-sensitive mutant of vaccinia IHD-W, designated is 9251, possesses a novel EcoRI restriction endonuclease site in the fragment D of the parental genome. Spontaneous ts+ revertants of ts 9251 fall into two distinct categories: the majority of revertants reacquired the parental EcoRI restriction profile, while one isolate maintained the mutant cleavage site. Using two-dimensional gel analysis of viral polypeptides induced by vaccinia virus in infected cytoplasms, it was observed that the fingerprint of mutant ts 9251 differed from the parental IHD-W in that a single viral protein of a molecular weight of 37,000 daltons migrated to a new isoelectric point. The revertants which had regained the wild-type restriction profile now encoded for a 37K polypeptide identical to that of the wild-type virus while the single revertant which maintained the novel EcoRI site possessed a 37K protein of a charge intermediate between ts 9251 and wild type. We conclude that ts 9251 can revert to the ts+ phenotype either by true reversion at the original mutant locus or by a second independent mutation within the same gene.
ASJC Scopus subject areas