An Inhibitor/anti-Inhibitor System Controls the Activity of Lytic Transglycosylase MltF in Pseudomonas Aeruginosa

Most bacterial cell envelopes contain a cell wall layer made of peptidoglycan. The synthesis of new peptidoglycan is critical for cell growth, division, and morphogenesis and is also coordinated with peptidoglycan hydrolysis to accommodate the new material. However, the enzymes that cleave peptidoglycan must be carefully controlled to avoid autolysis. In recent years, some control mechanisms have begun to emerge, although there are many more questions than answers for how most cell wall hydrolases are regulated. Here, we report a novel cell wall hydrolase control mechanism in Pseudomonas aeruginosa, which we discovered during our characterization of a mutant sensitive to the overproduction of a secretin protein. The mutation affected an uncharacterized Sel1-like repeat protein encoded by the PA3978 locus. In addition to the secretin-sensitivity phenotype, PA3978 disruption also increased resistance to a beta-lactam antibiotic used in the clinic. In vivo and in vitro analyses revealed that PA3978 binds to the catalytic domain of the lytic transglycosylase MltF and inhibits its activity. increment PA3978 mutant phenotypes were suppressed by deleting mltF, consistent with them having been caused by elevated MltF activity. We also discovered another interaction partner of PA3978 encoded by the PA5502 locus. The phenotypes of a increment PA5502 mutant suggested that PA5502 interferes with the inhibitory function of PA3978 toward MltF, and we confirmed that activity for PA5502 in vitro. Therefore, PA3978 and PA5502 form an inhibitor/anti-inhibitor system that controls MltF activity. We propose to name these proteins IltA (inhibitor A of lytic transglycosylase) and LiiA (lytic transglycosylase inhibitor A's inhibitor).