The Anti-Repressor MecR2 Promotes the Proteolysis of the mecA Repressor and Enables Optimal Expression of β-lactam Resistance in MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is an important human pathogen, which is cross-resistant to virtually all β-lactam antibiotics. MRSA strains are defined by the presence of mecA gene. The transcription of mecA can be regulated by a sensor-inducer (MecR1) and a repressor (MecI), involving a unique series of proteolytic steps. The induction of mecA by MecR1 has been described as very inefficient and, as such, it is believed that optimal expression of β-lactam resistance by MRSA requires a non-functional MecR1-MecI system. However, in a recent study, no correlation was found between the presence of functional MecR1-MecI and the level of β-lactam resistance in a representative collection of epidemic MRSA strains. Here, we demonstrate that the mecA regulatory locus consists, in fact, of an unusual three-component arrangement containing, in addition to mecR1-mecI, the up to now unrecognized mecR2 gene coding for an anti-repressor. The MecR2 function is essential for the full induction of mecA expression, compensating for the inefficient induction of mecA by MecR1 and enabling optimal expression of β-lactam resistance in MRSA strains with functional mecR1-mecI regulatory genes. Our data shows that MecR2 interacts directly with MecI, destabilizing its binding to the mecA promoter, which results in the repressor inactivation by proteolytic cleavage, presumably mediated by native cytoplasmatic proteases. These observations point to a revision of the current model for the transcriptional control of mecA and open new avenues for the design of alternative therapeutic strategies for the treatment of MRSA infections. Moreover, these findings also provide important insights into the complex evolutionary pathways of antibiotic resistance and molecular mechanisms of transcriptional regulation in bacteria.
Author Summary Methicillin-resistance Staphylococcus aureus (MRSA) is an important human pathogen, causing a wide range of infections. MRSA strains are resistant to virtually all β-lactam antibiotics and often are also resistant to many other classes of antibiotics, leaving physicians with few therapeutic options. MRSA is defined by the presence of the mecA gene. The induction of mecA transcription in response to β-lactams involves a unique series of proteolytic steps and some critical details of this signal transduction mechanism are still illusive. For instance, it is not fully explained why the induction of mecA by its cognate regulatory genes mecR1-mecI appears to be very inefficient and it is not clear if the observed MecI repressor proteolysis is mediated directly by the activated MecR1 sensor-inducer. In this study, we demonstrate that the mecA regulatory locus is not a two-component system but instead it is a three-component system containing the previously unrecognized anti-repressor mecR2 gene. MecR2 disturbs the binding of the repressor MecI to the mecA promoter, which leads to its proteolytic inactivation independently from MecR1. Moreover, our data shows that in the presence of functional mecR1-mecI genes, mecR2 is essential for a robust induction of mecA transcription and, as consequence, for the optimal expression of β-lactam resistance.