GLOBAL REGULATION OF EXPRESSION OF ANTIFUNGAL FACTORS BY A PSEUDOMONAS-FLUORESCENS BIOLOGICAL-CONTROL STRAIN

The root-colonizing bacterium Pseudomonas fluorescens BL915 protects a variety of seedlings from damping-off disease caused by the fungal pa thogen Rhizoctonia solani. Spontaneous pleiotropic mutants of P. fluor escens strain BL915 which fail to synthesize antifungal factors such a s chitinase, cyanide, and pyrrolnitrin and exhibit altered colony morp hology were isolated. Such mutants fail to inhibit the growth of R. so lani in vitro, and their biological control capability is sharply redu ced. We characterized a genomic DNA fragment from strain BL915 which, when introduced into these pleiotropic mutants, restored the lost func tions, the wild-type colony morphology, and biocontrol activity. DNA s equence analysis of the genomic fragment revealed the presence of gene s homologous to those of numerous bacterial global regulatory systems and identified a cluster of genes identical in organization to the Esc herichia coil gene cluster consisting of uvrY, uvrC, pgsA, and glyW Co ordinate biosynthesis of multiple antifungal products in some heterolo gous Pseudomonas strains in response to the introduction of the strain BL915 genomic fragment confirmed the regulatory nature of sequences c ontained on this fragment. Further genetic analysis indicated a gene h omologous to response regulators of bacterial two-component systems wa s sufficient to complement the pleiotropic mutants and to activate ant ifungal genes in heterologous strains. Marker exchange of a truncated version of this gene into the P. fluorescens BL915 chromosome generate d pleiotropic mutants indistinguishable from the original spontaneous mutants. Cloning and sequencing of the response regulator gene from se veral spontaneous mutants allowed identification of various nucleotide changes associated with the gene in such mutants.