We report here that PBP1a can dimerize but does not interact with PBP1b to form PBP1a/PBP1b het-erodimers in Escherichia coli. These findings support the idea of a relevant involvement of dimerization of both PBP1a and PBP1b during murein synthesis and suggest the existence of different peptidoglycan synthesis complexes. In eubacteria, the final steps in peptidoglycan synthesis are mediated by a set of periplasmic membrane-bound enzymes, the penicillin-binding proteins (PBPs) (12, 19, 24). In Esche-richia coli, 10 PBPs have been identified (8, 17, 21). Among them, only PBP1a and PBP1b are bifunctional enzymes orga-nized into two catalytic domains harboring transglycosylase and transpeptidase activities (13, 19, 22). PBP1c, a new mem-ber of these bimodular enzymes, has recently been shown to display transglycosylase activity, but it does not possess dem-onstrated transpeptidase activity and is unable to compensate for the loss of PBP1a and PBP1b (21). PBP1a consists of 85
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