Peptidoglycan recycling mediated by an ABC transporter in the plant pathogenAgrobacterium tumefaciens

Abstract

During growth and division, the bacterial cell wall peptidoglycan (PG) is remodelled, a process which results in the liberation of soluble PG fragments called muropeptides. These fragments can be released to the environment, where they can have far-reaching consequences in signalling and interspecies interactions, or reinternalized and recycled by the cell. Bacteria belonging to the Rhizobiales and Rhodobacterales groups of the Alphaproteobacteria lack the canonical muropeptide transporter AmpG, despite encoding orthologs of the other key PG recycling enzymes AmpD and NagZ in their genomes. Using high-throughput transposon sequencing (Tn-Seq), we identified an alternative ATP-binding-casette (ABC) family transporter, YejBEF-YepA, which takes over this role in the Rhizobiales phytopathogen Agrobacterium tumefaciens. The broad-spectrum peptide transporter "core" YejBEF functions with a PG-specific periplasmic substrate-binding protein (SBP) YepA to transport muropeptides into the cytoplasm for recycling. Muropeptide import by YejBEF-YepA governs the expression of the β-lactamase AmpC in A. tumefaciens through an AmpR repression system, and its absence confers hypersensitivity to β-lactam antibiotics. However, we show that the absence of YejBEF-YepA causes severe cell wall defects that go beyond the lowered AmpC activity. Thus, contrary to previously established Gram-negative models, PG recycling is vital for cell wall integrity in A. tumefaciens. YepA is widespread in the Alphaproteobacteria including in many clinically and environmentally relevant bacteria, suggesting that YejBEF-YepA-mediated PG recycling could represent an important but overlooked aspect of cell wall biology in the Alphaproteobacteria.