Identification of a new family of peptidoglycan transpeptidases reveals unusualcrosslinking is essential for viability inC. difficile

Most bacteria are surrounded by a peptidoglycan cell wall composed of glycan strands held together by short peptide crosslinks. There are two major types of crosslinks, termed 4-3 and 3-3 based on the amino acids involved. 4-3 crosslinks are created by penicillin-binding proteins (PBPs), while 3-3 crosslinks created byL,D-transpeptidases (LDTs). In well studied bacteria 3-3 crosslinks comprise only about 10% of the total and are not essential. However, in the opportunistic intestinal pathogenClostridioides difficile,about 70% of the crosslinks are 3-3. We show here that 3-3 crosslinks and LDTs are essential for viability inC. difficile. We also show thatC. difficilehas five LDTs, three with a YkuD catalytic domain as in all previously known LDTs and two with a VanW catalytic domain, whose function was until now unknown. The five LDTs exhibit extensive functional redundancy. VanW domain proteins are found in many Gram-positive bacteria but scarce in other lineages. We tested seven non-C. difficileVanW domain proteins and confirmed LDT activity in three cases. In summary, our findings uncover a new family of peptidoglycan crosslinking enzymes, assign a catalytic function to VanW domains, and demonstrate that 3-3 crosslinking is essential for viability inC. difficile, the first time this has been shown in any bacterial species. We compared the mRNA profile of WT C. difficile with that of our ∆ldt1∆ldt1∆ldt3 mutant to identify any change in gene expression between our mutant and WT