Location of N-acetylmuramyl-L-alanyl-D-glutamylmesodiaminopimelic acid, presumed signal molecule for beta-lactamase induction, in the bacterial cell.

Using a chromatographic method for the isolation and detection of periplasmic and cytoplasmic muropeptides avoiding radioactive labeling, we found that in the ampD-negative strain JRG582 the anhydromuropeptide N-acetylmuramyl-L-alanyl-D-glutamylmesodiaminopimelic acid (anhMurNAc tripeptide) accumulates not only in the cytoplasm but also in the periplasm. Simultaneously JRG582 carrying the Enterobacter cloacae genes ampC and ampR, which are necessary for the induction of beta-lactamase expression, overproduces beta-lactamase. We confirmed that the transmembrane protein AmpG transports a precursor muropeptide into the cytoplasm and that the formation of the anhMurNAc tripeptide takes place in the cytoplasm. anhMurNAc tripeptide can then be secreted into the periplasm. Therefore, the amount of anhMurNAc tripeptide in the cytoplasm is reduced not only by AmpD but also by transport out of the cell.