Abstract
For Staphylococcus aureus, it is hypothesized that two genes located upstream of the beta-lactamase gene, blaZ, are required for the inducible expression of beta-lactamase. blaR1 is predicted to encode a signal-transducing membrane protein, and blaI is predicted to encode a repressor protein. These same two genes may also regulate the production of penicillin-binding protein 2a (PBP 2a), a protein essential for expression of methicillin resistance. To confirm that these two genes encode products that can control both beta-lactamase and PBP 2a production, blaI, blaR1, and blaZ with a 150-nucleotide deletion at the 3' end were subcloned from a 30-kb staphylococcal beta-lactamase plasmid and three beta-lactamase-negative strains of methicillin-resistant S. aureus were transformed with the recombinant plasmid containing that insert. The production of PBP 2a and a nonfunctional beta-lactamase was detected by fluorography and by immunoblots with polyclonal antisera directed against each of the proteins. Whereas the parent strains did not produce beta-lactamase and constitutively produced PBP 2a, PBP 2a and a truncated beta-lactamase were now inducible in the transformants. Therefore, two plasmid-derived genes regulate the production of both PBP 2a and beta-lactamase.
Highlights
For Staphylococcus aureus, it is hypothesized that two genes located upstream of the 13-lactamase gene, blaZ, are required for the inducible expression of 1-lactamase. blaR1 is predicted to encode a signal-transducing membrane protein, and blal is predicted to encode a repressor protein
A second 3-lactam antibiotic resistance mechanism in staphylococci is production of penicillin-binding protein 2a (PBP 2a), which is unique to methicillin-resistant staphylococci [11]
3-lactam antibiotics in strains containing a 3-lactamase plasmid, and in most strains that have been examined, this inducibility of PBP 2a is lost when the t-lactamase plasmid is no longer present [16, 37]. mecA, the gene encoding PBP 2a, has been cloned [23] and sequenced [34], and a region of dyad symmetry immediately upstream of the structural gene is proposed to be an operator site where a repressor protein binds
Summary
For Staphylococcus aureus, it is hypothesized that two genes located upstream of the 13-lactamase gene, blaZ, are required for the inducible expression of 1-lactamase. blaR1 is predicted to encode a signal-transducing membrane protein, and blal is predicted to encode a repressor protein. BlaR1 is predicted to encode a signal-transducing membrane protein, and blal is predicted to encode a repressor protein These same two genes may regulate the production of penicillin-binding protein 2a (PBP 2a), a protein essential for expression of methicillin resistance. Like the blaRl in B. licheniformis [17, 18, 40], the deduced amino acid sequence of the staphylococcal blaRl predicts a membrane-spanning protein with an extracellular penicillin-binding domain and a cytoplasmic domain involved in signal transduction [38] This model of staphylococcal 3-lactamase regulation predicts that when BlaR1 is bound by the ,-lactam antibiotic, a signal leading to blaZ transcription is transduced through the membrane into the cell via its cytoplasmic domain. As predicted by the hypothesis, in all three transformed MRSA strains that contained blaRl and blaI the production of PBP 2a and ,-lactamase was regulated and induced by ,-lactam antibiotics
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