Nature of the plasmid-linked penicillinase regulatory region in Staphylococcus aureus

Summary

Four noninducible staphylococcal penicillinase plasmids reported to produce a very low basal level of penicillinase were investigated. Incorporation of 5-methyltryptophan, which is known to inactivate the operator binding site of wild-type penicillinase repressor and thereby elicit penicillinase synthesis, did not induce penicillinase synthesis in any of these “micro” mutants. Therefore, these plasmids are not simply peni ^S mutants. Heterodiploid strains composed of a plasmid fully constitutive for penicillinase synthesis and one of the various “micro” penicillinase plasmids were constructed. Three of these heterodiploids produce a normal basal level of penicillinase and are inducible by 5-methyltryptophan but not by the standard gratuitous penicillinase inducer. Therefore, each of these three noninducible “micro” plasmids produces a peni ^S repressor, but in addition, each must bear a mutation in the penZ region. The fourth heterodiploid produces a fully constitutive level of penicillinase. The noninducible “micro” plasmid present in this heterodiploid must contain a penI ⁻ mutation and a mutation in the penZ region. Consequently, each of these four noninducible “micro” plasmids contains at least two mutants genes. Hence, the phenotype of noninducibility plus low basal penicillinase is not due to a point mutation in a second penicillinase regulatory region as has been proposed. Instead, these results strongly suggest that there is only one penicillinase regulatory gene located on the penicillinase plasmid and that this gene (penI) specifies the penicillinase repressor.