Staphylococcus aureus is among the most prevalent and antibiotic-resistant of pathogenic bacteria. The resistance of S. aureus to prototypal -lactam antibiotics is conferred by two mechanisms: (i) secretion of hydrolytic -lactamase enzymes and (ii) production of -lactam-insensitive penicillin-binding proteins (PBP2a). Despite their distinct modes of resistance, expression of these proteins is controlled by similar regulation systems, including a repressor (BlaI/MecI) and a multidomain transmembrane receptor (BlaR1/MecR1). Resistance is triggered in response to a covalent binding event between a -lactam antibiotic and the extracellular sensor domain of BlaR1/MecR1 by transduction of the binding signal to an intracellular protease domain capable of repressor inactivation. This study describes the first crystal structures of the sensor domain of BlaR1 (BlaR S ) from S. aureus in both the apo and penicillin-acylated forms. The structures show that the sensor domain resembles the -lactam-hydrolyzing class D -lactamases, but is rendered a penicillin-binding protein due to the formation of a very stable acyl-enzyme. Surprisingly, conformational changes upon penicillin binding were not observed in our structures, supporting the hypothesis that transduction of the antibiotic-binding signal into the cytosol is mediated by additional intramolecular interactions of the sensor domain with an adjacent extracellular loop in BlaR1.The evolution and dissemination of antibiotic resistance in pathogenic bacteria are a growing concern. Many of the antimicrobial "wonder drugs" society has come to rely on for the treatment of bacterial infections have been neutralized by new strains equipped with a variety of molecular countermeasures. Methicillin-resistant Staphylococcus aureus is a prominent example of this (1-3). Although -lactam antibiotics such as penicillin have long been the drugs of choice for infections of S. aureus, current therapies for methicillin-resistant S. aureus infections now depend on distinct antibiotic classes such as glycopeptides to counter the increased resistance to penicillin and its derivatives. The recent emergence of glycopeptide-resistant strains of S. aureus (4, 5) underscores the need not only for the development of novel therapeutics, but for better understanding of the molecular mechanisms involved in antibiotic resistance.-Lactam antibiotics kill bacteria by inhibiting the cell wall transpeptidases (also known as penicillin-binding proteins (PBPs) 1 ) that are responsible for the essential cross-linking of peptidoglycan during synthesis of the rigid bacterial cell wall. Resistance to -lactam antibiotics in S. aureus can be conferred by two mechanisms. In one case, -lactamase enzymes are secreted from the bacterium to hydrolytically inactivate -lactam antibiotics. Resistance of this form is encoded by the blaZ gene and is carried by Ͼ95% of S. aureus isolates (3). The second resistance mechanism is expression of PBP2a, a cell wall transpeptidase with broad-spectrum insensitivity to -lacta...