Further studies on the induction of mutation in Haemophilus influenzae by N-methyl-N'-nitro-N-nitrosoguanidine: lack of an inducible error-free repair system and the effect of exposure medium.

Haemophilus influenzae is shown to lack the inducible, error-free repair system for alkylation damage that others have found in Escherichia coli. Prior growth in a low concentration of N-methyl-N'-nitro-N-nitrosoguanidine had only an additive effect on a subsequent brief exposure to a high concentration. Furthermore, chloramphenicol did not significantly modify the mutagenic response. In both respects, H. influenzae differs from E. coli. Experiments carried out in preparation for these tests showed that exposure to N-methyl-N'-nitro-N-nitrosoguanidine in complex growth medium was more effective by about an order of magnitude than exposure in pH 6.0 tris--maelare buffer in inducing mutations, in killing the cells, and in causing strand breaks in the preexisting DNA and gaps in newly synthesized DNA. Thus the effect of the medium is on the amount of initial damage rather than on some special feature of the mutation process. Part but not all of the effect can be accounted for by the difference in pH of the 2 media. The nature of the mutagenic process is the same under the 2 exposure conditions; i.e., reparable pre-mutational damage is produced by the agent and subsequently converted to final mutation by replication. The dose--effect curves have a non-linear initial portion under both exposure conditions, and possible reasons for this non-linearity are discussed.