Characterization of the murMN operon involved in the synthesis of branched peptidoglycan peptides in Streptococcus pneumoniae.

The murMN operon, recently identified in the genome of Streptococcus pneumoniae, encodes for enzymes involved in the synthesis of branched structured muropeptides in the pneumococcal peptidoglycan; inactivation of murMN causes production of a peptidoglycan composed exclusively of linear muropeptides and a virtually complete loss of resistance in penicillin-resistant strains (Filipe, S. R., and Tomasz, A. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 4891-4896). The experiments described in this paper follow up these observations. Primer extension analysis was used to identify the putative promoter region of the murMN operon in penicillin-susceptible and -resistant strains. Selective inactivation of the murN gene in the penicillin-resistant strain Pen6 caused production of an unusual peptidoglycan that contained only single amino acid residues in the muropeptide branches, indicating that the product of murN was involved with the addition of the second amino acid and the product of murM was involved with the addition of the first amino acid (alanine or serine) to the peptidoglycan cross-bridge. Allelic replacement of the mosaic murM gene of strain Pen6 with murM of the penicillin-susceptible laboratory strain caused enrichment of the peptidoglycan in linear muropeptides. The findings suggest that the genetic determinant primarily controlling the synthesis of branched muropeptides in the pneumococcal peptidoglycan is murM.