Functional analysis of Streptococcus pneumoniae MurM reveals the region responsible for its specificity in the synthesis of branched cell wall peptides

The recently identified murMN operon of Streptococcus pneumoniae encodes enzymes involved in the synthesis of branched structured muropeptides of the pneumococcal cell wall peptidoglycan. Its inactivation was shown to cause production of a peptidoglycan composed exclusively of linear muropeptides and a virtually complete loss of resistance in penicillin-resistant strains. The studies described in this communication follow up these observations in several directions. The substrate of the MurM-catalyzed reaction (addition of alanine or serine) was identified as the lipid-linked N-acetylglucosamine-muramyl pentapeptide. Different murM alleles from several penicillin-resistant S. pneumoniae strains, each with a characteristic branched peptide pattern, were cloned into pLS578, a pneumococcal plasmid capable of replicating in S. pneumoniae, and transformed into the penicillin-susceptible laboratory strain R36A All transformants remained penicillin-susceptible; however, their cell wall composition changed in directions corresponding to the muropeptide pattern of the strain from which the murM allele was derived. This suggests that the muropeptide composition of the pneumococcal cell walls is determined by the particular murM allele carried by the cells. A 30-amino acid long sequence within the MurM protein was shown to be the main determinant of the specificity of the reaction (addition of alanine versus serine).