MecI represses synthesis from the beta-lactamase operon of Staphylococcus aureus

Plasmid diploids were constructed in Staphylococcus aureus to study the eff ect of the repressor of methicillin resistance (Mecl) on the synthesis of b oth beta-lactamase and the beta-lactamase repressor (Blal). Mecl-mediated r epression of the synthesis of beta-lactamase was shown by reduction in the specific activity of nitrocefinase in bacteria containing a plasmid carryin g mecl but not when containing the same plasmid deleted for mecl. Antisera prepared against purified Mecl and against purified Blal were used in Weste rn blots which showed that Mecl repressed the synthesis of Blal in these di ploids. The interactions between the mec operon and the bla operon are disc ussed.