Essentiality of LD-Transpeptidation in Agrobacterium tumefaciens

Peptidoglycan (PG), a mesh-like structure which is the primary component of the bacterial cell wall, is crucial to maintain cell integrity and shape. While most bacteria rely on penicillin binding proteins (PBPs) for crosslinking, some species employ LD-transpeptidases (LDTs). Unlike PBPs, the essentiality and biological functions of LDTs remain largely unclear. The Hyphomicrobiales order of the Alphaproteobacteria, known for their polar growth, have PG which is unusually rich in LD-crosslinks, suggesting that LDTs may play a more significant role in PG synthesis in these bacteria. Here, we investigated LDTs in the plant pathogen Agrobacterium tumefaciens and found that LD-transpeptidation, resulting from at least one of 14 putative LDTs present in this bacterium, is essential for its survival. Notably, a mutant lacking a distinctive group of 7 LDTs which are broadly conserved among the Hyphomicrobiales exhibited reduced LD-crosslinking and tethering of PG to outer membrane {beta}-barrel proteins. Consequently, this mutant suffered severe fitness loss and cell shape rounding, underscoring the critical role played by these Hyphomicrobiales-specific LDTs in maintaining cell wall integrity and promoting elongation. Tn-sequencing screens further revealed non-redundant functions for A. tumefaciens LDTs. Specifically, Hyphomicrobiales-specific LDTs exhibited synthetic genetic interactions with division and cell cycle proteins, and a single LDT from another group. Additionally, our findings demonstrate that strains lacking all LDTs except one displayed distinctive phenotypic profiles and genetic interactions. Collectively, our work emphasizes the critical role of LD-crosslinking in A. tumefaciens cell wall integrity and growth and provides insights into the functional specialization of these crosslinking activities.