Bacteriophage T4 ν mutants with a slow rate of thymine-dimer excision and a particular reactivation phenotype

Bacteriophage T4 uvs52 is a member of a class of UV-sensitive mutants with UV survival between T4 wild-type and v mutants. The mutation promotes recombination between extracellularly UV-irradiated phages. However, the location is adjacent to, or in, gene v. The question whether uvs52 is a v mutant with a particular type of v gene expression was investigated with acid solubilization of ¹⁴C]thymine dimers from DNA incubated with extracts from T4-infected cells. The dimer-removal activity of extracts from uvs52-infected cells was half that of wild-type T4, and similar to that of the v am5 and v op14 enzymes induced in the appropriate su⁺ hosts. The initial velocity of incision of UV-irradiated DNA by partially purified extracts from cells infected with uvs52 was 15% of that of the wild-type. Excision activity was not disturbed in such extracts. Further evidence of the location of uvs52 in gene v followed from the negative results from complementation assays with mixtures of extracts from cells infected with uvs52, uvs21 (another member of this class) or v1.

The relation between initial incision activity and substrate concentration (UV-irradiated ¹⁴C-DNA) suggested that the uvs52 endonuclease V is mutant with a high affinity and a slow rate of thymine-dimer incision. The reactivation phenotype was explained by assuming a slow rate of dimer excision in vivo as well, continuing throughout the reproductive cycle of the phage and leading to intermediate UV sensitivity and photoreactivability. The increased recombination frequencies were explained by assuming that the single-stranded regions of the DNA produced by incisions made at the end of the reproductive cycle are readily recombined into the growing DNA pool.