Identification of a type III secretion system in Lysobacter enzymogenes strain C3: Molecular characterization and functional analysis of the pathway as an integral part in bacterial-fungal interactions.

摘要

A genetic basis for pathogenicity is known as the type III secretion system (TIIISS) in both animal and plant pathogenic bacteria. Pathogenic bacteria employ this system to deliver proteinaceous virulence factors into the host cell. In animal as well as plant pathogenic systems, the delivered proteins are believed to affect normal processes of the host cell. In this work, a TIIISS has been identified in Lysobacter enzymogenes strain C3, a bacterium demonstrated earlier to be used in biological control of fungi. Annotation of the pathway suggests that it is structurally similar to Group I plant pathogenic bacteria, comprised mainly of Pseudomonas spp. However, the pathway is distinct in that the genes are high in G+C% content (>72%), and homology is not restricted to those reported exclusively belonging to plant pathogenic bacteria. In addition, the gene cluster possesses a physically-linked putative sensor kinase regulator and is interspersed with several open reading frames with no homology to any defined TIIISS genes.; Real-time PCR assays indicate increased expression of the ATPase pathway gene, sctN, in nutrient-limiting conditions. Basal levels of expression are indicated when the bacterium is grown in nutrient-rich tryptic soy broth. Real-time PCR assays also show a negligible increase in sctN gene expression when the bacterium is grown in the presence of yeast cell walls and Magnaporthe poae hyphae.; Mutational analyses of several genes within the pathway did not produce any discernable phenotype when the interactions between the mutants and M. poae were microscopically viewed. Conversely, a deletion of the pathway produced a phenotypic change. This mutation attenuated biofilm formation and polar attachment to the hyphae. A similar phenotype was also observed when an ORF of unknown function was deleted. The observed loss of polar attachment led to the designation of this ORF as the f&barbelow;ungal attachment A&barbelow; (fatA) gene.; This work has initially defined a TIIISS, its expression in nutrient-limiting conditions, and one of its functions as an integral part of the pathogenic interaction between strain C3 and M. poae. Furthermore, this is the first TIIISS that has been described in bacteria as directed at a fungal host.