Gene unstabilization induced by heat and by nitrous acid.

Previous communications have dealt with induction of mutations by heating bacterial cells in aqueous suspensions (Zamenhof, 1958a; Zamenhof and Greer, 1958) or in the dry state in vacuum (Zamenhof, 1960), or by treating the cells with nitrous acid (Zamenhof, 1958b). The present paper deals with cases in which the above agents have induced well defined gene unstabilization. The term "gene unstabilization" denotes a phenomenon in which a gene mutates to one with a higher spontaneous rate of mutation. Examples of this phenomenon are: (i) the mutation of a stable enzyme producer to an unstable one, which mutates to enzyme nonproducer with a high spontaneous mutation rate, and (ii) the mutation of a stable prototroph to an auxotroph, which is unstable and reverts to prototrophy with a high spontaneous mutation rate. Induced mutants in both these categories will be described in this paper. The "unstable gene" will be arbitrarily designated as the one with a spontaneous mutation rate higher than 1 per 104 cells per generation. Studies of such high mutabilities were avoided in the past, partially because of the lack of techniques.