Mutation fixation in MNNG-treated Haemophilus influenzae as determined by transformation

Abstract

A transformation assay has been used to follow the fixation of mutations to novobiocin resistance induced by N -methyl- N ′-nitro- N -nitrosoguanidine (MNNG) in Haemophilus influenzae . Very few mutations are produced by recently treated DNA, but many are produced by the DNA from cells that have been incubated for a time after exposure to MNNG. The time course of this mutation fixation is shown to coincide reasonably well with the time course of semiconservative DNA synthesis, as judged by uptake studies and by isopycnic centrifugation of density-labeled cells. Incubation with bromodeoxyuridine (BrdUrd) during the fixation period decreases the number of mutations that are fixed, showing in another way the importance of DNA synthesis for fixation. Mutations fixed in the presence of BrdUrd are not more sensitive to 313-nm radiation than those fixed in its absence, suggesting that these residual mutations are fixed in the absence of extensive DNA replication. Mutations newly fixed in the absence of BrdUrd are much more sensitive to 313-nm radiation than are the same mutations some cell generations later. This shows that the newly fixed mutations are in a state that is different from their final form, either because they are in regions of DNA with special configurations of the strands or because they are in a region of DNA that is a hybrid between an old, alkylated strand and a new strand with some bases different from normal. The data suggest that it is unlikely that anything like all the mutations that are fixed in H. influenzae arise by direct action of MNNG on the replication fork. Many of the results can be explained in terms of fixation during semiconservative replication of premutational lesions, some of which are initially located some distance from the replication fork. The final yield would then depend on the relative rates of removal of the lesions by repair and of fixation by replication.