On the lack of host-cell reactivation of UV-irradiated phage T5. I. Interference of T5 infection with the host-cell reactivation of phage T1.

UV-irradiated phage T5, in contrast to T1, T3 and T7, fail to display host-cell reactivation (HCR) when infecting excision-repair proficient Escherichia coli cells. Possible causes of this lack of HCR (which T5 shares with the T-even phages) have been investigated by studying HCR of T1 under conditions of superinfection by T5. Repair-proficient B/r cells were infected at low multiplicity with UV-irradiated phage T1 in the presence of 1.8 mg/ml caffeine and were superinfected after 15 min with heavily UV-irradiated T5 amber mutants at highly multiplicity. The caffeine, which is later diluted out, prevents any T1 repair prior to T5 superinfection, and UV (254 nm) irradiation of T5 with 144 J/m2 reduces the ability of this phage to exclude T1, thus permitting a reasonable fraction of the mixedly infected complexes to produce T1 progeny. Under these conditions, T5 superinfection causes loss of HCR in about 90% of the T1-producing complexes. Superinfection with unirradiated T5 likewise inhibits HCR of T1, but superinfection with irradiated T3 (a host-cell reactivable phage) does not. This indicates that the observed HCR inhibition of T1 results from T5 infection rather than from competition of irradiated foreign DNA for the excision-repair enzymes of the bacterial host. Employment of appropriate T5 amber mutants has shown that "first-step transfer" (FST) of T5 DNA (involving only 8% of the T5 genome) is sufficient for HCR inhibition, but that transfer of the remainder DNA in addition inhibits a previously described minor T1 recovery process. HCR inhibition of T1, and thus presumably lack of HCR in T5 itself, is ascribed to a substance which is produced either post infection by a gene located in the FST segment of the T5 genome, or which is transferred from extracellular T5 together with the FST DNA.