RtpA, a gene encoding a bacterial two-component sensor kinase, determines pathogenic traits of Pseudomonas tolaasii, the causal agent of brown blotch disease of a cultivated mushroom, Pleurotus ostreatus

Abstract

Pseudomonas tolaasii strain PT814 produces extracellular toxins, tolaasins, and a volatile toxin, tovsin, that are responsible for the induction of brown blotch and rotting, respectively, in a cultivated mushroom, Pleurotus ostreatus. Insertions of single transposon mini-Tn 5km 1 into the chromosome of P. tolaasii strain PT814 generated mutants that are pleiotropically defective in tolaasin and protease production, and altered in colony morphology. The mutants, however, produce tovsin at the level of wild-type. Variants phenotypically similar to the pleiotropic mutants of P. tolaasiistrain PT814 spontaneously occurred in P. tolaasii strain S8501 at 22–30°C in vitro. The occurrence of variants was significantly reduced in the presence of extracts of P. ostreatus or at a temperature of 15–20°C The rtpA gene (rtpA = regulator gene of tolaasin production and other pleiotropic traits) isolated from a P. tolaasii strain PT814 gene library restored the wild-type phenotype in both the mini-Tn5 km 1 insertion and spontaneous mutants, mini-Tn5 km 1 insertions were also located in the allele of rtpA. Nucleotide sequencing of the rtpA DNA revealed an open reading frame of 2,751 bp predicted to encode a protein consisting of 917 amino acid residues with a molecular mass of 100.6 kDa and displaying the conserved amino acid sequence of both sensor and receiver domains of “bacterial two-component regulators.” The data suggest that the machinery responding to environmental stimuli is essential for the pathogenic interaction of P. tolaasii with the mushroom.