Two dd -Carboxypeptidases from Mycobacterium smegmatis Affect Cell Surface Properties through Regulation of Peptidoglycan Cross-Linking and Glycopeptidolipids

During the peptidoglycan (PG) maturation of mycobacteria, the glycan strands are interlinked by both 3-3 (between twomeso -diaminopimelic acids [meso -DAPs]) and 4-3 cross-links (betweend -Ala andmeso -DAP), though there is a predominance (60 to 80%) of 3-3 cross-links. Thedd -carboxypeptidases (dd -CPases) act on pentapeptides to generate tetrapeptides that are used byld -transpeptidases as substrates to form 3-3 cross-links. Therefore,dd -CPases play a crucial role in mycobacterial PG cross-link formation. However, the physiology ofdd -CPases in mycobacteria is relatively unexplored. In this study, we deleted twodd -CPase genes,msmeg_2433 andmsmeg_2432 , both individually and in combination, fromMycobacterium smegmatis mc2 155. Though the singledd -CPase gene deletions had no significant impact on the mycobacterial physiology, many interesting functional alterations were observed in the double-deletion mutant,viz ., a predominance in PG cross-link formation was shifted from 3-3 cross-links to 4-3, cell surface glycopeptidolipid (GPL) expression was reduced, and susceptibility to β-lactams and antitubercular agents was enhanced. Moreover, the survival rate of the double mutant within murine macrophages was higher than that of the parent. Interestingly, the complementation with any one of thedd -CPase genes could restore the wild-type phenotype. In a nutshell, we infer that the altered ratio of 4-3 to 3-3 PG cross-links might have influenced the expression of surface GPLs, colony morphology, biofilm formation, drug susceptibility, and subsistence of the cells within macrophages.IMPORTANCE The glycan strands in mycobacterial peptidoglycan (PG) are interlinked by both 3-3 and 4-3 cross-links. Thedd -CPases generate tetrapeptides by acting on the pentapeptides, andld -transpeptidases use tetrapeptides as substrates to form 3-3 cross-links. In this study, we showed that simultaneous deletions of twodd -CPases alter the nature of PG cross-linking from 3-3 cross-links to 4-3 cross-links. The deletions subsequently decrease the expression of glycopeptidolipids (significant surface lipid present in many nontuberculous mycobacteria, includingMycobacterium smegmatis ) and affect other physiological parameters, like cell morphology, growth rate, biofilm formation, antibiotic susceptibility, and survival within murine macrophages. Thus, unraveling the physiology ofdd -CPases might help us design antimycobacterial therapeutics in the future.