A T7 amber mutant defective in DNA-binding protein.

A T7 amber mutant, UP-2, in the gene for T7 DNA-binding protein was isolated from mutants that could not grow on sup+ ssb-1 bacteria but could grow on glnU ssb-1 and sup+ ssb+ bacteria. The mutant phage synthesized a smaller amber polypeptide (28,000 daltons) than T7 wild-type DNA-binding protein (32,000 daltons). DNA synthesis of the UP-2 mutant in sup+ ssb-1 cells was severely inhibited and the first round of replication was found to be repressed. The abilities for genetic recombination and DNA repair were also low even in permissive hosts compared with those of wild-type phage. Moreover, recombination intermediate T7 DNA molecules were not formed in UP-2 infected nonpermissive cells. The gene that codes for DNA-binding protein is referred to as gene 2.5 since the mutation was mapped between gene 2 and gene 3.