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Antimicrob. Agents Chemother. doi:10.1128/AAC.00677-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Mutational Analysis of Class A and Class B Penicillin-Binding Proteins in Streptococcus gordonii

Department of Fundamental Microbiology, University of Lausanne, Switzerland

^* To whom correspondence should be addressed. Email: philippe.moreillon{at}unil.ch.

	Abstract

High molecular-weight (HMW) penicillin-binding proteins (PBPs) are divided into class A and class B PBPs, which are bifunctional transpeptidases-transglycosylases and monofunctional transpeptidases, respectively. We determined the sequence of the HMW PBP genes of Streptococcus gordonii, a gingivo-dental commensal related to Streptococcus pneumoniae. Five HMW PBPs were identified - including 3 class A (PBP 1A, 1B and 2A) and 2 class B (PBP 2B and 2X) - by homology with those of S. pneumoniae, and by radiolabelling with [³H]-penicillin. Single and double-deletion of each of them were achieved by allelic replacement. All could be deleted, except for PBP 2X that was essential. Morphological alterations occurred after deletion of PBP 1A (lozenge shape), PBP 2A (separation defect and chaining) and PBP 2B (aberrant septation and premature lysis), but not PBP 1B. The muropeptide cross-link pattern remained similar in all strains, indicating that cross-linkage by one missing PBP could be replaced by others. However, PBP 1A mutants presented shorter glycan chains (by 30%) and a relative decrease (25%) of one monomer stem-peptide. Growth rate and viability under aeration, hyper-osmolarity and penicillin-exposure were affected primarily in PBP 2B-deleted mutants. In contrast, chain-forming PBP 2A-deleted mutants withstood better aeration, probably because they formed clusters that impaired oxygen diffusion. Double-deletion could be generated in any PBP combination and resulted in more altered mutants. Thus, single deletion of 4 of the 5 HMW genes had a detectable effect on the bacterial morphology and/or physiology, and only PBP 1B seemed redundant a priori.

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