Studies on mechanisms for the maintenance of constant mutability: mutability and the resistance to mutagens.

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Abstract
Several mechanisms for the maintenance of a constant mutability most advantageous for the species have been discussed, and evidence for one such mechanism has been presented. Two mutants of Escherichia coli 15s more resistant to the mutagen N-methyl-N[image]-nitro-N-nitrosoguanidine (NTG) exhibited lower NTG-induced mutability (azide resistance, total auxotrophs) than the parent strain; the higher the NTG resistance, the lower the induced mutability. At least one of these resistant mutants, selected for by the presence of NTG, had in the presence of this potent mutagen approximately the same mutability as the parent strain in the absence of mutagen, due to the above compensation (decrease in induced mutability). The phenomenon is reversible: upon removal of NTG, the parent strain tends to supersede the resistant strain, again without a change in mutability of the system. Spontaneous mutability is also considerably lower for NTG-resistant mutants. These mutants were also more resistant to UV, suggesting that a similar mechanism (DNA repair) accounts here for higher NTG-survival and for lower NTG-induced mutability, as well as for lower spontaneous mutability.