Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2001
Publication Date: N/A
Interpretive Summary: Staphylococcus aureus is an important human pathogen. It is the causative agent of a variety of infections, ranging from superficial skin infections to more serious systemic infections. Beta-lactam antibiotics, such as penicillin and newer classes of related antibiotics, which were once able to cure infections caused by this bacterium are no longer effective. This bacterium has become resistant to all beta-lactams by acquiring novel resistance genes from other bacteria. These resistance genes allow beta-lactam- resistant S. aureus to survive in the presence of these antibiotics, thus leaving only a limited option for successful treatment of infections caused by this bacterium. The gene, mecA, that confers a wide array of beta-lactam resistance in S. aureus, enables resistant cells to make a protein (PBP2a) that can perform cell wall synthesis even in the presence of very high levels of beta-lactams. Moreover, additional genes (mecI-mecR1) provide resistant cells the mechanism to increase the amount of PBP2a in the presence of beta-lactams. Other genes, such as blaZ, that allows the synthesis of protein beta- lactamase to destroy penicillin, and genes (blaR1-blaI) that regulate blaZ expression, can also influence the level of mecA. Thus, the understanding of the mechanism by which these two sets of regulatory genes (mecR1-mecI and blaR1-blaI) control the synthesis of PBP2a might pave the way to develop alternative strategies to control infections caused by beta-lactam-resistant staphylococci.
Technical Abstract: In staphylococci mecA and blaZ, genes encoding a penicillin binding protein (PBP2a) and beta-lactamase, respectively, confer resistance to beta-lactams. The mec and bla regulators, mecR1-mecI and blaR1- bla1, encode inducer-repressors with sufficient amino acid homology to suggest that they could co-regulate PBP2a production. In order to test this hypothesis, plasmids containing mec and bla regulatory sequences were introduced into Staphylococcus aureus containing a chromosomal mecA-lacZ transcriptional fusion. Co-repression was confirmed by demonstrating a gene dosage-dependent reduction in beta- galactosidase activity by either MecI or BlaI and additive repression when both were present. MecI-MecI, BlaI-BlaI, and MecI-BlaI interactions were demonstrated in the yeast two-hybrid assay. Although purified MecI and BlaI protected the mec promoter-operator (P-O) sequences, protection was achieved by MecI at one-third the concentration required for BlaI. In addition, MecI was approximately 3-fold more effective at mecA transcriptional repression than was BlaI, and MecI had twice the affinity of BlaI for mec P-O binding sites. While MecI and BlaI were interchangeable as repressors, there was a marked difference between MecR1 and BlaR1 in the rate and specificity of induction of chromosomal mecA transcription. Induction through BlaR1 by a beta-lactam (CBAP) was 10-fold greater than through MecR1 at 60 min, and was 81% of maximal by 2 hrs while induction through MecR1 never exceeded 20% of maximal. Furthermore, MecI-mediated mecA transcriptional repression could not be relieved by BlaR1, demonstrating the repressor-specificity of induction.