Mutation induction in Haemophilus influenzae by ICR-191. II. Role of DNA replication and repair

Evidence is presented to show that presumptive frameshift mutations induced in Haemophilus influenzae by ICR-191 are fixed very rapidly, essentially at the time of treatment. DNA synthesis during treatment is essential for fixation, but DNA synthesis after treatment has no effect. The conclusion is drawn that the mutagen acts at the replication fork, possibly to stabilize misannealings arising in association with the discontinuities in the newly synthesized DNA. These results agree with earlier results on Escherichia coli showing that ICR-191 produces peak mutation frequencies in synchronized cultures at times when the replication fork has reached the locus being studied. They are in sharp contrast to the earlier results in H. influenzae with nitroso compounds and hydrazine that suggest these agents produce randomly distributed, reparable pre-mutational damage that still can be fixed (converted to final mutation) for some time after treatment when the replication fork reaches them. No evidence for such persistent pre-mutational lesions was found with ICR-191. A defect in incision appeared to have very little influence on mutation induction by ICR-191 though it caused much more lethality. The interpretation of the mutation data was made somewhat uncertain, however, by an unexplained plating-density effect on the expression of the mutants in this strain. In contrast, incision deficiencies in E. coli and Salmonella typhimurium have been reported to cause a large increase in mutation induction and to allow lesions at some distance from the replication fork to produce mutations.