A Colicin M-Type Bacteriocin from Pseudomonas aeruginosa Targeting the HxuC Heme Receptor Requires a Novel Immunity Partner

Abstract

Pyocins are bacteriocins secreted by , and they assist in the colonization of different niches. A major subset of these antibacterial proteins adopt a modular organization characteristic of polymorphic toxins. They include a receptor-binding domain, a segment enabling membrane passage, and a toxin module at the carboxy terminus, which eventually kills the target cells. To protect themselves from their own products, bacteriocin-producing strains express an immunity gene concomitantly with the bacteriocin. We show here that a pyocin equipped with a phylogenetically distinct ColM toxin domain, PaeM4, mediates antagonism against a large set of isolates. Immunity to PaeM4 is provided by the inner membrane protein PmiC, which is equipped with a transmembrane topology not previously described for the ColM family. Given that strains lacking a gene are killed by PaeM4, the presence of such an immunity partner likely is a key criterion for escaping cellular death mediated by PaeM4. The presence of a TonB box in PaeM4 and enhanced bacteriocin activity under iron-poor conditions strongly suggested the targeting of a TonB-dependent receptor. Evaluation of PaeM4 activities against TonB-dependent receptor knockout mutants in PAO1 revealed that the heme receptor HxuC (PA1302) serves as a PaeM4 target at the cellular surface. Because other ColM-type pyocins may target the ferrichrome receptor FiuA, our results illustrate the versatility in target recognition conferred by the polymorphic nature of ColM-type bacteriocins. The antimicrobial armamentarium of a bacterium is a major asset for colonizing competitive environments. Bacteriocins comprise a subset of these compounds. Pyocins are an example of such antibacterial proteins produced by , killing other strains. A large group of these molecules show a modular protein architecture that includes a receptor-binding domain for initial target cell attachment and a killer domain. In this study, we have shown that a novel modular pyocin (PaeM4) that kills target bacteria via interference with peptidoglycan assembly takes advantage of the HxuC heme receptor. Cells can protect themselves from killing by the presence of a dedicated immunity partner, an integral inner membrane protein that adopts a transmembrane topology distinct from that of proteins currently known to provide immunity against such toxin activity. Understanding the receptors with which pyocins interact and how immunity to pyocins is achieved is a pivotal step toward the rational design of bacteriocin cocktails for the treatment of infections.

An integrated model system to gain mechanistic insights into biofilm formation and antimicrobial resistance development inPseudomonas aeruginosaMPAO1

“…Comparative genomics with the PAO1 type strain uncovered an inventory of conserved and strain-specific genes, and a list of genome-wide SNPs, extending an earlier study that had compared a subset of genomic regions [20]. Among the 232 MPAO1-unique gene clusters, bacteriocins [53] were enriched, which play a role in restricting the growth of closely related microbial competitors to gain an advantage in colonizing a variety of niches [54]. The complete MPAO1 genome enabled us to remap valuable existing Tn-seq datasets from relevant conditions [24], thereby identifying 39 MPAO1 essential genes that had escaped detection so far due to reference-based PAO1 mapping.…”

Diversity of Contact-Dependent Growth Inhibition Systems of Pseudomonas aeruginosa

“…The Pseudomonas bacteriocin Pyocin S2 binds the ferripyoverdine receptor FpvA (45,46), while pyocin S5 uses the ferripyochelin receptor FptA for uptake (47). In addition, the lectin-like bacteriocin LlpA interacts with specific residues on extracellular loop 6 of BamA to facilitate intoxication (48), and the colicin M-like bacteriocin PaeM4 uses the heme receptor HxuC (49). These proteins are possible candidates for CdiA receptors in P. aeruginosa; however, further work is needed to determine the exact receptors used by different P. aeruginosa CdiA proteins.…”

Lectin-Like Bacteriocins

“…As cargo genes, these llpA genes may spread via horizontal gene transfer. Lectin-like bacteriocin genes are hallmarked by a lower than host-average G + C content, a feature they share with modular bacteriocins ( Mavrodi et al, 2009 ; Loper et al, 2012 ; Dingemans et al, 2016 ; Ghequire et al, 2017a , b ; Ghequire and Öztürk, 2018 ). In general no clear correlation between LlpA phylogeny and Pseudomonas taxonomy can be made (except for P. aeruginosa , Supplementary Figure S2 ).…”