Identification and characterization of novel cell wall hydrolase CwlT: a two-domain autolysin exhibiting n-acetylmuramidase and DL-endopeptidase activities

A cell wall hydrolase homologue, Bacillus subtilis YddH (renamed CwlT), was determined to be a novel cell wall lytic enzyme. The cwlT gene is located in the region of an integrative and conjugative element (ICEBs1), and a cwlT-lacZ fusion experiment revealed the significant expression when mitomycin C was added to the culture. Judging from the Pfam data base, CwlT (cell wall lytic enzyme T (Two-catalytic domains)) has two hydrolase domains that exhibit high amino acid sequence similarity to dl-endopeptidases and relatively low similarity to lytic transglycosylases at the C and N termini, respectively. The purified C-terminal domain of CwlT (CwlT-C-His) could hydrolyze the linkage of d-gamma-glutamyl-meso-diaminopimelic acid in B. subtilis peptidoglycan, suggesting that the C-terminal domain acts as a dl-endopeptidase. On the other hand, the purified N-terminal domain (CwlT-N-His) could also hydrolyze the peptidoglycan of B. subtilis. However, on reverse-phase HPLC and mass spectrometry (MS) and MS-MS analyses of the reaction products by CwlT-N-His, this domain was determined to act as an N-acetylmuramidase and not a lytic transglycosylase. Moreover, the site-directed mutagenesis analysis revealed that Glu-87 and Asp-94 are sites related with the cell wall lytic activity. Because the amino acid sequence of the N-terminal domain of CwlT exhibits low similarity compared with those of the soluble lytic transglycosylase and muramidase (goose lysozyme), this domain represents "a new category of cell wall hydrolases."