Abstract
The active site serine residue of penicillin-binding protein 3 of Escherichia coli that is acylated by penicillin (Ser-307) has been converted to a cysteine residue using a simple and efficient two primer method of site-directed mutagenesis. The resulting thiol-penicillin-binding protein 3 was expressed under the control of the lacUV5 promoter in a high copy number plasmid. Constitutive expression of the thiol-enzyme (but not of the wild-type enzyme) was lethal, and the plasmid could only be maintained in E. coli strains that carried the lacIq mutation. Induction of the expression of the thiol-enzyme resulted in inhibition of cell division and the growth of the bacteria into very long filamentous cells. The inhibition of septation was probably due to interference of the function of the wild-type penicillin-binding protein 3 in cell division by the enzymatically inactive thiol-enzyme, and this implies that penicillin-binding protein 3 acts as part of a complex in vivo. We were unable to detect any acylation of the thiol-enzyme by penicillin, but it is not yet clear if this was because the thioester was not formed at an appreciable rate, or if it was formed but was too unstable to be detected by a modified penicillin-binding protein assay.