Distinct colicin M-like bacteriocin-immunity pairs in Burkholderia

The Escherichia coli bacteriocin colicin M (ColM) acts via degradation of the cell wall precursor lipid II in target cells. ColM producers avoid self-inhibition by a periplasmic immunity protein anchored in the inner membrane. In this study, we identified colM-like bacteriocin genes in genomes of several β-proteobacterial strains belonging to the Burkholderia cepacia complex (Bcc) and the Burkholderia pseudomallei group. Two selected Burkholderia ambifaria proteins, designated burkhocins M1 and M2, were produced recombinantly and showed antagonistic activity against Bcc strains. In their considerably sequence-diverged catalytic domain, a conserved aspartate residue equally proved pivotal for cytotoxicity. Immunity to M-type burkhocins is conferred upon susceptible strains by heterologous expression of a cognate gene located either upstream or downstream of the toxin gene. These genes lack homology with currently known ColM immunity genes and encode inner membrane-associated proteins of two distinct types, differing in predicted transmembrane topology and moiety exposed to the periplasm. The addition of burkhocins to the bacteriocin complement of Burkholderia reveals a wider phylogenetic distribution of ColM-like bacteriotoxins, beyond the γ-proteobacterial genera Escherichia, Pectobacterium and Pseudomonas, and illuminates the diversified nature of immunity-providing proteins.