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Pseudomonas strains produce a variety of molecules with antagonistic activity against other microorganisms. While bacteriocins kill related bacteria, antifungal activity is mediated by various non-protein compounds, such as high-affinity siderophores and diverse antibiotics like phenazines, phloroglucinols, pyoluteorin, pyrrolnitrin and lipopeptides. However, only few Pseudomonas secondary metabolites selectively targeting related bacteria are known. This research focused on the characterization of novel antibacterial activities of Pseudomonas species directed at related bacteria, in particular phytopathogenic pseudomonads and xanthomonads. Through screening of a collection of Pseudomonas rhizosphere isolates for such antagonistic activities, two strains with unusual antimicrobial spectrum were selected for further study.The rice rhizosphere isolate Pseudomonas putida RW10S1 selectively targets other Pseudomonas strains including the opportunistic human pathogen Pseudomonas aeruginosa and several phytopathogens. This remarkable intra-genus antagonistic activity is mediated by a surface colonization-promoting secondary metabolite, promysalin, that is composed of salicylic acid and 2,8-dihydroxymyristamide bridged by a unique 2-pyrroline-5-carboxyl moiety. To our knowledge, no comparable (antimicrobial) molecules have been identified in other organisms. Therefore, it is not surprising that the 15.7-kb promysalin biosynthetic gene cluster lacks an equivalent in other bacteria. This cluster composed of 15 genes seems to have evolved from a mosaic gene assembly recruited from different metabolic pathways in diverse, mostly phylogenetically distant or unrelated organisms. Gene product functional predictions combined with identification of some intermediates revealed a pathway involving several biosynthetic activities for recruiting precursors from fatty acid and proline metabolism and converting these into unusual building blocks. Notably, one biosynthetic gene appears to encode a prokaryotic member in the BAHD family of plant-specific acyltransferases, and may have evolved from a horizontally acquired plant gene. The production of promysalin is not triggered by low-iron availability, as is the case for salicylic acid-containing siderophores, but is dependent on the Gac/Rsm global regulatory system that controls various aspects of social behavior in γ-Proteobacteria. Being produced by a rice root-associated Pseudomonas strain, promysalin may contribute to aggressive colonization of structured environments, such as the nutrient-enriched rhizosphere of plants, in competition with other pseudomonads.The banana rhizosphere isolate P. putida BW11M1 displays broad antagonistic activity against Xanthomonas pathovars, including the major rice pathogen Xanthomonas oryzae pv. oryzae. This antagonism is not mediated by its lectin-like bacteriocin but apparently involves two unrelated types of secondary metabolites with a different but partially overlapping anti-Xanthomonas spectrum. Biosynthesis of a first compound, designated xantholysin, involves three non-ribosomal peptide synthetase (NRPS) genes organized in two separate genomic clusters (approximately 46 kb). The corresponding NRPSs comprise two, eight and four modules respectively. In silico domain analysis of these NRPSs, combined with initial structure determination, revealed that xantholysin belongs to a new group of cyclic lipopeptides with a 14-amino acid peptide moiety that was established recently by discovery of entolysin from the insect pathogen Pseudomonas entomophila. Xantholysin also promotes swarming motility and biofilm formation by strain BW11M1, and has antifungal and hemolytic activity.The gene cluster required for production of a second anti-Xanthomonas activity is flanking the xantholysin-initiatory NRPS gene and shares several features with the biosynthetic gene cluster of the phytotoxin toxoflavin produced by Burkholderia glumae. This suggests that strain BW11M1 produces additional anti-Xanthomonas compounds related to 7-azapteridine antibiotics, probably with modifications of the core structure.

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