Lactococcus lactis Gene yjgB Encodes a γ- d -Glutaminyl- l -Lysyl- Endopeptidase Which Hydrolyzes Peptidoglycan

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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.