[Show abstract][Hide abstract] ABSTRACT: YjgB is one of five peptidoglycan hydrolases previously identified in Lactococcus lactis. Analysis of its amino acid sequence revealed that YjgB contains an NlpC/P60 domain, whereas no specific cell wall binding
domain or motif could be identified. The NlpC/P60 family is characterized by three conserved residues, a cysteine, a histidine,
and a polar residue. In agreement with the presence of a Cys residue in the catalytic site of YjgB, its enzymatic activity
was enhanced in the presence of dithiothreitol. Peptidoglycan-hydrolyzing activity of YjgB was detected in growing cells of
an L. lactis strain overexpressing YjgB, as revealed by the presence of disaccharide (DS)-dipeptide in the muropeptide composition of
the overexpressing strain. YjgB hydrolyzes the peptide chains of L. lactis muropeptides between γ-d-Gln and l-Lys residues. Its hydrolytic activity was detected on DSs with tetra- and pentapeptide chains, whereas hydrolytic activity
was very low on DS-tripeptides. Thus, we demonstrated that YjgB is an endopeptidase which cleaves γ-d-Gln-l-Lys bonds in peptide chains of L. lactis peptidoglycan.
[Show abstract][Hide abstract] ABSTRACT: The peptidoglycan hydrolase (PGH) complement of Lactococcus lactis was identified by amino acid sequence similarity searching of the L. lactis IL-1403 complete genome sequence. Five PGHs that are not encoded by prophages were detected, including the previously characterized AcmA and AcmB proteins. Four of these PGHs, AcmA to AcmD, contain a catalytic domain homologous to that of enterococcal muramidase, but they have different domain structures. The fifth one (YjgB) has sequence similarity with the active-site domain of peptidoglycan-specific endopeptidases. The three new PGH-encoding genes identified in this study are all actively transcribed in L. lactis subsp. cremoris MG1363. The relative abundance of their transcripts varied during growth and was maximal during the early exponential growth phase. The three encoded proteins have peptidoglycan-hydrolyzing activities which are detected only at acidic pHs by zymography. Like AcmA and AcmB, AcmC has N-acetylglucosaminidase activity rather than the N-acetylmuramidase activity predicted by sequence similarity.