Autorepression of ethidium bromide mutagenesis in Kluyveromyces lactis, a petite-negative yeast.

mannose or galactose, susceptibility of the culture to ethidium bromide mutagenesis was influenced by the degree of respiratory repression imposed by the sugar in question. Thus the more repressive the sugar, the greater the rate of mutation. We have now tested the effect of alternative, non-utilizable catabolite-repressing sugars, such as mannitol, sorbose and xylose, used at high concentrations, but insufficient to cause growth lag or a considerable decrease in growth rate. Cells were previously grown in 0.5% glucose-containing medium, to minimize catabolite repression, and transferred to ethidium bromide-containing media of 0.5 % glucose plus a non-utilizable sugar (Fig. 2b). The results indicate that such substances, to a lesser or greater degree, interfere with ethidium bromide-induced petite mutation; xylose (3 %, w/v) was particularly effective although less so than glucose itself (total concentration lo%, w/v). It seems likely that this behaviour is associated with the degree of catabolite repression imposed. Hence it is concluded that one component at least, of the protection of the p-factor afforded by glucose against mutagenesis, is explicable in terms of repression of respiratory activity and that other non-utilizable catabolite repressors can mimic the effect of glucose in this regard. Further, it is interesting to consider that, although pre-conditioning of a culture to 5 % (w/v) glucose raises susceptibility to mutagenesis in repressing regimes (Hammond et al., 1974), the converse is true for cells pre-conditioned with 0.5 % glucose. Finally, whereas the degree of glucose repression of cells before drug treatment had some effect as far as susceptibility to ethidium bromide petite mutagenesis was concerned (Fig. la), the presence or absence of the unmetabolizable sugar in the pre-growth medium was of no consequence in the time-course of mutagenesis.