Polymorphism of an essential outer-membrane protein determines killing by Pseudomonas lectin-like bacteriocins

Pseudomonads are equipped with an arsenal of antagonism-mediating molecules to gain hold in competitive environments. A major subset of molecules contributing to this process are secreted antibacterial proteins or bacteriocins, and these compounds typically kill neighboring strains with a narrow-spectrum of activity. Bacteriocins are currently under renewed interest for medical applications. Their high selectivity, efficacy and biodegradability make them a promising drug lead for cocktail development. One class of such allelopathic molecules, suitable for large-scale production in planta, are composed of a tandem of sequence-diverged B-lectin domains. These so-called lectin-like bacteriocins or LlpAs have been mainly studied in Pseudomonas and kill neighboring cells after attachment to D-rhamnose residues present in the common polyantigen of lipopolysaccharide, via an unknown mechanism of action. In this study genomes of spontaneous LlpA-resistant mutants were analyzed for the presence of single nucleotide polymorphisms. We demonstrate that LlpA-susceptible strains become fully resistant to bacteriocin action by mutations in one of three surface-exposed moieties of an essential β-barrel outer membrane protein. Interestingly, polymorphism of this loop in different Pseudomonas species groups is correlated to LlpA susceptibility and bacteriocin-targeted cells all share an identical of highly similar signature motif in this loop. Since heterologous expression of this receptor confers LlpA susceptibility upon resistant strains, this outer-membrane protein likely represents the primary bacteriocin selectivity determinant in pseudomonads. Contrary to modular bacteriocins that require uptake via the Tol or Ton system to gain access to target cells, the identification of an essential outer-membrane protein as a LlpA receptor is indicative of a novel bacteriocin killing mechanism, likely initiated by active impairment of a receptor at the cellular surface.status: publishe