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 en zymes involved in the synthesis of branched structured muropeptides of the pneumococcal cell wall peptidoglycan. Its inactivation was shown to cause p roduction of a peptidoglycan composed exclusively of linear muropeptides an d 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-acetylglucosami ne-muramyl pentapeptide. Different murM alleles from several penicillin-res istant S. pneumoniae strains, each with a characteristic branched peptide p attern, were cloned into pLS578, a pneumococcal plasmid capable of replicat ing in S. pneumoniae, and transformed into the penicillin-susceptible labor atory strain R36A All transformants remained penicillin-susceptible; howeve r, their cell wall composition changed in directions corresponding to the m uropeptide pattern of the strain from which the murM allele was derived. Th is suggests that the muropeptide composition of the pneumococcal cell walls is determined by the particular murM allele carried by the cells. A 30-ami no acid long sequence within the MurM protein was shown to be the main dete rminant of the specificity of the reaction (addition of alanine versus seri ne).