SUMMARY: A small proportion of spontaneous penicillinase-negative mutants of the penicillinase-magno-constitutive strain 569/ prove to be highly unstable and revert to the relatively stable magno-constitutive parent phenotype at a high rate spontaneously. If incubated at 45° or at the normal growth temperature of 35° with chloramphenicol (20 μg./ml.) the organisms of a broth culture of this meta-stable negative strain can be induced to undergo 100% conversion to the fully de-repressed magno-constitutive state within 90 to 120 min. The conversion event appears to occur at random amongst the bacilli, whether single or joined in pairs; there is little or no phenotypic lag between committal to reproduction in the derepressed state and full expression of gene potential in the form of maximal rate of penicillinase production and a 1000-fold increase in penicillin resistance. Induced conversion is inhibited by nalidixic acid (10 μg./ml.).

It is concluded that the conversion event, for which some possible mechanisms are discussed, is controlled by a thermo-labile protein inhibitor.


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