Binding of the gene repressor BlaI to the bla operon in methicillin-resistant Staphylococcus aureus

Abstract

The expression of the gene products in many methicillin-resistant Staphylococcus aureus (MRSA) strains is regulated by the gene repressor BlaI. Here we show that BlaI is a mixture of monomer and dimer at in vivo concentrations, binds to the operator regions preferentially as a monomeric protein, and the measured dissociation constants and in vivo concentrations account for the basal level transcription of the resistance genes. These observations for the first time provide a quantitative picture of the processes that take place in the cytoplasm that lead to the induction of antibiotic resistance factors to counter the challenge by β-lactams.

Figure 1

The sensor and repressor proteins of the bla operon are involved in the onset of β-lactam resistance in MRSA.

Figure 2

Fluorescence anisotropy measurement of BlaI binding to DNA. The lines show the corresponding fits to the model shown in Scheme 1, where the equilibrium between BlaI monomer and dimer is characterized by K_d. The equations used are described in Supporting Information. The panel on the right is the same as the image on the left panel, except that the plot is given for a more narrow concentration range.

Scheme 1

Model for BlaI binding to DNA. Designations M and D denote BlaI monomer and dimer, respectively.