Bacteria often have a major type-5 PBP which represents the most abundant LMM PBP they produce. The most highly expressed PBP in listerial membranes is PBP5. In a previous study we confirmed that PBP5 is a DD-carboxypeptidase that preferentially degrades low-molecular-weight substrates [11]. In the present study we found that PBP5 is also a protein with a high affinity for β-lactams. L. monocytogenes produces one more type-5 PBP – Lmo2812 – but its role in cell wall biosynthesis and catalytic activity had not previously been examined. In

this study, recombinant Lmo2812 was expressed in E. coli and purified in order to characterize its enzymatic activity and role in cell physiology. Lmo2812 lacking its signal sequence was expressed as a His-tagged fusion protein in Quisinostat cost the cytoplasm of E. coli, which allowed the EPZ-6438 concentration purification of large amounts of functionally active protein. Type-5 PBPs, with the exception of S. aureus PBP4, are strict DD-carboxypeptidases and are unable to catalyze transpeptidation reactions [19]. Using the synthetic triEpigenetics inhibitor peptide Nα,Nε-Diacetyl-Lys-D-Ala-D-Ala and the natural monomer NAcGlc-NAcMur-pentapeptide in an in vitro assay, we showed that Lmo2812 displays weak DD-carboxypeptidase activity, cleaving the peptide bond between the subterminal and terminal D-alanine moieties. However, the recombinant Lmo2812 was active against neither E. coli peptidoglycan

nor the natural dimeric muropeptide D45 (disaccharide pentapeptide disaccharide tetrapeptide). This suggests that Lmo2812, like PBP5 [11], preferentially Phospholipase D1 degrades low-molecular-weight substrates. Analysis of the muropeptide profiles of a L. monocytogenes mutant demonstrated that the lack of Lmo2812 activity does not affect the muropeptide structure of its peptidoglycan. However, the ratio of pentapeptides to tripeptides was found to be increased in cells lacking both Lmo2812 and PBP5. Similar changes have been observed in the peptidoglycan from a L. monocytogenes mutant lacking PBP5 [12], B. subtilis devoid of PBP5 [18] and S. pneumoniae with disrupted PBP3 activity [22]. These changes in the muropeptide profile

indicate that L. monocytogenes PBP5, like PBP5 of B. subtilis and PBP3 of S. pneumoniae, is a DD-carboxypeptidase that plays a basic role in the maturation of the cell wall peptidoglycan. Mutations in genes coding for low molecular mass PBPs are not lethal for the bacterial cell and in general these proteins seem to be redundant. Mutants can survive not only the lack of individual LMM PBPs, e.g. Pseudomonas aeruginosa [23], S. pneumoniae [24], S. aureus [25] and Myxococcus xanthus [26], but also the loss of all LMM PBPs, e.g. E. coli [27], Neisseria gonorrhoeae [28] and B. subtilis [29]. Similarly, we demonstrated that the inactivation of L. monocytogenes genes lmo2812 and lmo2754 is not lethal and these gene products are dispensable for the growth and survival of the cells.