Transcription of the gene mediating methicillin resistance in Staphylococcus aureus (mecA) is corepressed but not coinduced by cognate mecA and beta-lactamase regulators

Resistance to beta -lactam antibiotics in staphylococci is mediated by mecA and blaZ, genes encoding a penicillin-binding protein (PBP2a) with low bet a -lactam affinity and beta -lactamase, respectively. The mec and bla regul ators, mecR1-mecI and blaR1-blaI, respectively, encode inducer-repressors w ith sufficient amino acid homology to suggest that they could coregulate PB P2a production. In order to test this hypothesis, plasmids containing mec a nd bla regulatory sequences were introduced into Staphylococcus aureus cont aining a chromosomal rnecA-lacZ transcriptional fusion. Compression was con firmed by demonstrating a gene dosage-dependent reduction in beta -galactos idase activity by either MecI or Mal and additive repression when both were present. Both MecI-MecI and BlaI-BlaI homodimer and MecI-BlaI heterodimer interactions were demonstrated in the yeast two-hybrid assay, and purified MecI and BlaI protected the same mec promoter-operator sequences. However, MecI was approximately threefold more effective at mecA-lacZ transcriptiona l repression than was Mal. While MecI and BlaI displayed similar activity a s repressors of mecA transcription, there was a marked difference between M ecR1 and BlaR1 in the rate and specificity of induction. Induction through BlaR1 by a beta -lactam was 10-fold greater than through MecR1 at 60 min an d was 81% of maximal by 2 h, while induction through MecR1 never exceeded 2 0% of maximal. Furthermore, complementation studies showed that MecI- or Bl aI-mediated mecA transcriptional repression could be relieved by induction through homologous but not heterologous sensor-inducer proteins, demonstrat ing the repressor specificity of induction.