Identification of gene loci controlling pectate lyase production and soft-rot pathogenicity in Pseudomonas marginalis

Pseudomonas marginalis is an important postharvest pathogen capable of causing soft rot in a wide variety of harvested fruits and vegetables. Following transposon mutagenesis, we isolated two groups of P. marginalis CY091 mutants deficient in production of pectate lyase (Pel) and soft-rot pathogenicity in plants. The first group, designated Pel-, was caused by the insertion of Tn5 into a pel structural gene, and the second group, designated LemA-, was caused by the insertion of Tn5 into a regulatory locus corresponding to the lemA gene previously identified in other Gram-negative bacteria. The LemA- mutants also exhibited alteration in colony morphology and showed deficiency in production of protease (Prt). A cosmid clone pCIC carrying the P. marginalis lemA gene was isolated and characterized. pCIC was capable of restoring Pel production and soft-rot pathogenicity in LemA- mutants of P. marginalis and Pseudomonas viridiflava, indicating that the function of lemA gene in these two pseudomonads was similar and interchangeable. Using MudI-mediated mutagenesis, we isolated a third group of P. marginalis mutants deficient in production of Pel, Prt, and soft-rot pathogenicity. Mutants in this group (designated GacA-1) contained an insertion of MudI in a locus corresponding to the gacA gene of P. viridiflava. Like LemA- mutants, GacA- mutants also exhibited alteration in colony morphology and showed deficiency in production of Pel and Prt. However, GacA- mutants produced much lower levels of levan and fluorescent pyoverdine siderophore than the wild type and LemA- mutants. These results provide the first genetic evidence that P. marginalis produces a single alkaline Pel for maceration of plant tissue and demonstrate that production of Pel, Prt, levan, and pyoverdin by this bacterium is mediated by the two-component lemA/gacA gene system.