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. Overall design: 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