A link in transcription between the native pbpB and the acquired mecA gene in a strain of Staphylococcus aureus.

Conditional mutants of pbpB with an IPTG-inducible promoter were used to compare the effects of interrupted transcription of this gene in a meticillin-sensitive (MSSA) and a meticillin-resistant (MRSA) strain of Staphylococcus aureus. After 3 h growth following the removal of IPTG, multiplication of the MSSA strain stopped abruptly, cells began to lyse, and membrane preparations showed greatly decreased quantities of penicillin-binding protein (PBP) 2. In contrast, the MRSA strain continued to grow for at least 20 h in the IPTG-free medium, but with gradually increasing doubling times, which eventually reached 180 min. The peptidoglycan produced during this period of extremely slow growth showed only minor alterations, but cells with abnormal morphology accumulated in the culture, the abundance of mecA transcript gradually declined, and the cellular amounts of PBP2A were significantly decreased. Adding back the IPTG inducer caused rapid resumption in the transcription of pbpB, followed by an increase in the transcription of mecA. No changes were detected in the transcription of pbpA, C and D, the determinant of 16S rRNA or the housekeeping gene pta. Promoter fusion experiments suggested that the transcription of the resistance gene mecA may respond to some regulatory signal generated in the bacteria during changes in the transcription of pbpB.