Mutation induction by MNNG in a bacteriophage of Haemophilus influenzae

Abstract Three temperature-sensitive mutants of the Haemophilus influenzae phage HP1c1 were tested for reversion to wild type (ts → ts + ). Treatment with N -methyl- N ′-nitro- N -nitrosoguanidine (MNNG) produced revertants at levels up to 0.1% of the total progeny phage from treated lysogens. Cells treated with MNNG after infection with whole ts phage produced progeny phage with similar reversion frequencies, but when the uninfected cells or the phage were treated alone no reversion was induced. Fixation of premutational lesions was shown to occur with no evidence for host-cell DNA synthesis, indicating that phage DNA synthesis may be responsible for fixation of mutation in phage DNA. Evidence is given which shows that prophage DNA replicating by the cells' replicating system after treatment and before induction, produces the same number of revertants per survivor as phage DNA which is replicated outside the host genome. Two of the phage mutants ( ts 1 and ts2 ) reversted at similar frequencies, while one of the mutants ( ts3 ) exhibited a much lower induced reversion frequency.