An amino acid substitution in penicillin-binding protein 3 creates pointed polar caps in Escherichia coli

Abstract

The pbpB gene product penicillin-binding protein 3 (PBP3) of Escherichia coli is one of the major targets of beta-lactam antibiotics. At the permissive temperature, the temperature-sensitive pbpBr1 mutant, which was obtained after selection for increased resistance to cephalexin, shows a dramatic change in shape which has never been observed before; the polar caps are pointed. We show that the substitution of amino acid Asn-361 by Ser, previously shown to be responsible for increased cephalexin resistance and for temperature sensitivity, causes the pointed polar caps. However, comparison of the morphological and physiological characteristics of the pbpBr1 mutant with those of other pbpB mutants suggests that the formation of pointed polar caps is not correlated with temperature sensitivity or cephalexin resistance. Partial inactivation of PBP3 by subinhibitory concentrations of cephalexin, furazlocillin, and piperacillin resulted in the formation of slightly pointed polar caps, suggesting that the shape of the polar caps is correlated with PBP3 activity. The large change in the shape of the polar caps was accompanied by a small change in the kinetics of peptidoglycan synthesis and in the local rate of surface synthesis activity along the cell envelope.