A potential space making role for a lytic transglycosylase in cell wall biogenesis revealed by a beta-lactamase induction phenotype in Pseudomonas aeruginosa

Pseudomonas aeruginosa encodes the beta-lactamase AmpC, which promotes resistance to beta-lactam antibiotics. Expression of ampC is induced by fragments of the peptidoglycan (PG) cell wall released upon beta-lactam treatment. These drugs target transpeptidase enzymes that form cell wall crosslinks. However, they do not block the activity of the transglycosylases that polymerize the glycan chains. Thus, drug-treated cells produce uncrosslinked PG polymers that have been shown to be rapidly degraded in the related gram-negative bacterium Escherichia coli. This degradation is performed by enzymes called lytic transglycosylases (LTs), which generate the anhydro-muropeptide (AMP) products sensed by the AmpR regulator that activates ampC expression. To identify factors required for proper PG biogenesis in P. aeruginosa, we used a reporter gene fusion to the ampC promoter to screen for mutants induced for ampC expression in the absence of drug. To our surprise, we found that inactivation of SltB1, an LT enzyme expected to produce the AMP products required for ampC induction, counterintuitively led to elevated ampC expression. This induction required another LT enzyme called MltG, suggesting that inactivation of SltB1 reduces the efficiency of PG crosslinking, causing the degradation of a subset of nascent PG strands by MltG to generate the inducing signal. Our results therefore support a model in which SltB1 uses its LT activity to open space in the PG matrix for the efficient insertion of new material, a function commonly thought to be restricted to endopeptidases that cut cell wall crosslinks. IMPORTANCEInducible beta-lactamases like the ampC system of Pseudomonas aeruginosa are a common determinant of beta-lactam resistance among gram-negative bacteria. The regulation of ampC is elegantly tuned to detect defects in cell wall synthesis caused by beta-lactam drugs. Studies of mutations causing ampC induction in the absence of drug therefore promise to reveal new insights into the process of cell wall biogenesis in addition to aiding our understanding of how resistance to beta-lactam antibiotics arises in the clinic. In this study, an ampC induction phenotype for a mutant lacking an enzyme that cleaves cell wall glycans was used to uncover a potential role for glycan cleavage in making space in the wall matrix for the insertion of new material during cell growth.