Transcription of the Gene Mediating Methicillin Resistance in Staphylococcus aureus ( mecA ) Is Corepressed but Not Coinduced by Cognate mecA and β-Lactamase Regulators

Abstract

Resistance to β-lactam antibiotics in staphylococci is mediated by mecA and blaZ, genes encoding a penicillin-binding protein (PBP2a) with low β-lactam affinity and β-lactamase, respectively. The mec andbla regulators, mecR1-mecI andblaR1-blaI, respectively, encode inducer-repressors with sufficient amino acid homology to suggest that they could coregulate PBP2a production. In order to test this hypothesis, plasmids containingmec and bla regulatory sequences were introduced into Staphylococcus aureus containing a chromosomal mecA-lacZ transcriptional fusion. Corepression was confirmed by demonstrating a gene dosage-dependent reduction in β-galactosidase activity by either MecI or BlaI 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 atmecA-lacZ transcriptional repression than was BlaI. While MecI and BlaI displayed similar activity as repressors ofmecA transcription, there was a marked difference between MecR1 and BlaR1 in the rate and specificity of induction. Induction through BlaR1 by a β-lactam was 10-fold greater than through MecR1 at 60 min and was 81% of maximal by 2 h, while induction through MecR1 never exceeded 20% of maximal. Furthermore, complementation studies showed that MecI- or BlaI-mediatedmecA transcriptional repression could be relieved by induction through homologous but not heterologous sensor-inducer proteins, demonstrating the repressor specificity of induction.