Cloning and characterization of PBP1C, a third member of the multimodular class A penicillin-binding proteins of Escherichia coli

All proteins of Escherichia coli that covalently bind penicillin have been cloned except for the penicillin-binding protein (PBP) 1C, For a detailed understanding of the mode of action of beta-lactam antibiotics, cloning of the gene encoding PBP1C was of major importance. Therefore, the structural gene was identified in the E. coli genomic lambda library of Kohara and subcloned, and PBP1C was characterized biochemically. PBP1C is a close homologue to the bifunctional transpeptidases/transglycosylases PBP1A and PBP1B and likewise shows murein polymerizing activity, which can be blocked by the transglycosylase inhibitor moenomycin. Covalently linked to activated Sepharose, PBP1C specifically retained PBP1B and the transpeptidases PBP2 and -3 in addition to the murein hydrolase MltA. The specific interaction with these proteins suggests that PBP1C is assembled into a multienzyme complex consisting of both murein polymerases and hydrolases, Overexpression of PBP1C does not support growth of a PBP1A(tS)/PBP1B double mutant at the restrictive temperature, and PBP1C does not bind to the same variety of penicillin derivatives as PBPs 1A and 1B. Deletion of PBP1C resulted in an altered mode of murein synthesis. It is suggested that PBP1C functions in vivo as a transglycosylase only.