Identification and characterization of a locus which regulates multiple functions in Pseudomonas tolaasii, the cause of brown blotch disease of Agaricus bisporus

Abstract

Pseudomonas tolaasii, the causal agent of brown blotch disease of Agaricus bisporus, spontaneously gives rise to morphologically distinct stable sectors, referred to as the phenotypic variant form, at the margins of the wild-type colonies. The phenotypic variant form is nonpathogenic and differs from the wild type in a range of biochemical and physiological characteristics. A genomic cosmid clone (pSISG29) from a wild-type P. tolaasii library was shown to be capable of restoring a range of characteristics of the phenotypic variant to those of the wild-type form, when present in trans. Subcloning and saturation mutagenesis analysis with Tn5lacZ localized a 3.0-kb region from pSISG29, designated the pheN locus, required for complementation of the phenotypic variant to the wild-type form. Pseudomonas tolaasii causes the economically important brown blotch disease of the mushroom Agaricus bisporus (Lange) Imbach (53). Synthesis of the low-molecular-weight (M r , 1,985) lipodepsipeptide extracellular toxin tolaasin by P. tolaasii is primarily responsible for eliciting disease symptoms (6,38,39). Tolaasin causes disruption of cell membranes from a range of cell types and has both ion channel-forming and biosurfactant properties (6,26,38). The ability of P. tolaasii to show a positive chemotactic response to A. bisporus exudates (20) and its ability to attach to mycelial surfaces (9, 35, 37) may also be important as steps in the early stages of development of brown blotch disease.P. tolaasii undergoes phenotypic variation, a strategy frequently used by bacteria to survive disparate environmental conditions. Old colonies of P. tolaasii frequently produce sectors which, when subcultured onto fresh medium, show different phenotypically distinct colonial forms (10, 29, 36). The wild-type strain is opaque, mucoid, pathogenic, and nonfluorescent, whereas the sectors are translucent, nonmucoid, nonpathogenic, and fluorescent (10, 57). In addition to alteration in virulence, qualitative analysis has revealed that a number of biochemical properties are also changed in the transition from the wild type to a phenotypic variant. These include the loss of the ability to produce tolaasin, hydrolyze casein, and grow normally on medium containing cetrimide (10). Finally, the variant form swims faster and shows a more rapid chemotactic response than the wild type (20). Phase change from the pathogenic to the nonpathogenic form in P. tolaasii has been referred to as a smooth-to-rough transition (10). This description generally designates a difference in the lipopolysaccharide (LPS) content of the two forms (16,29). Recent studies by Hutchison (25) have demonstrated that there is no difference in the LPS profiles of the smooth and rough forms of P. tolaasii. Therefore, the terms ''wild type'' and ''phenotypic variant'' are used here instead of ''smooth'' and ''rough,'' respectively.The aim of this study was to identify the molecular switch controlling the transition from the wild type to the phenotypic variant in P. tolaasii. First, we ...

Analysis of the Role of recA in Phenotypic Switching of Pseudomonas tolaasii

“…Colonies of the wildtype strain of P. tolaasii (designated 1116S) are opaque, mucoid, pathogenic, and nonfluorescent and produce tolaasin (6). In contrast, colonies of the stable phenotypic variant form of 1116S (designated 1116R) are translucent, nonmucoid, nonpathogenic, and fluorescent and do not produce tolaasin (6).…”

The global activator GacA of Pseudomonas aeruginosa PAO positively controls the production of the autoinducer N‐butyryl‐homoserine lactone and the formation of the virulence factors pyocyanin, cyanide, and lipase

“…The cognate sensor appears to be the transmembrane protein LemA (Hrabak and Willis, 1992;Rich et al, 1994). Both the gacA and lemA genes are highly conserved in fluorescent pseudomonads (Liao et al, 1994;Corbell and Loper, 1995;Grewal et al, 1995). In Salmonella typhimurium, the gacA homologue sirA is essential for invasion and virulence (Johnston et al, 1996), and a gacA homologue is also known to be present in Escherichia coli (Moolenaar et al, 1987) and in Erwinia carotovora (Eriksson et al, 1996).…”

SirA Orthologs Affect both Motility and Virulence

“…Genetic evidence suggests that the SirA orthologs of Salmonella, Erwinia, and Pseudomonas are also phosphorylated by proteins orthologous to BarA of E. coli. The BarA ortholog is known as BarA in Salmonella, ExpA in Erwinia, and GacS, LemA, or PheN in Pseudomonas (7,12,22,29,31,42,50,56,57). For simplicity, the barA orthologs of all species (expA and lemA/gacS/pheN) will be referred to as barA throughout this report.…”