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United States Department of Agriculture

Agricultural Research Service


item Whistler, C
item Corbell, N
item Ream, W
item Sarniguet, A
item Loper, Joyce

Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Pseudomonas fluorescens Pf-5 is a biological control agent that inhabits root surfaces and protects roots from infection by fungi and bacteria that cause plant diseases. Pf-5 also produces many antibiotics that are toxic to plant-pathogenic bacteria and fungi, and these antibiotics are important in biological control of plant diseases. Pf-5 does not produce these antibiotics all the time, however; instead, it produces the antibiotics only during some portions of its life cycle and under certain environmental conditions. Our hypothesis is that biocontrol of plant disease can be more successful if the antibiotics needed to suppress plant disease are expressed under the full spectrum of environmental conditions where disease occurs. Therefore, we are studying how the production of antibiotics are regulated in Pf-5. In this paper, we describe our experiments evaluating the interactions of three genes that regulate antibiotic production in Pf-5. We demonstrate that the product of one gene is controlled by the other two genes. We also show that the three genes control the response of Pf-5 to an environmental stress, and thereby demonstrate that antibiotic production and stress response are regulated by common mechanisms in this bacterium. These results provide one piece in our emerging picture of how biocontrol activity is regulated, and also provide information needed for future attempts to enhance the consistent efficacy of biological control.

Technical Abstract: Three global regulators are known to control antibiotic production by Pseudomonas fluorescens. A two-component regulatory system comprised of the sensor kinase GacS (previously called ApdA or LemA) and GacA, a member of the FixJ family of response regulators, is required for antibiotic production. A mutation in rpoS, which encodes the stationary-phase sigma factor, differentially affects antibiotic production and reduces the capacity of stationary-phase cells of P. fluorescens to survive exposure to oxidative stress. The gacA gene of P. fluorescens strain Pf-5 was isolated and the influence of gacS and gacA on rpoS transcription, levels, and oxidative stress response of Pf-5 was determined. A gacA mutant of Pf-5 was selected that contained a single nucleotide substitution within a predicted alpha-helical region, which is highly-conserved among the FixJ family of response regulators. At the entrance to stationary phase, Rpos content in gacS- and gacA- mutants of Pf-5 was less than 20% of the wild-type level. Transcription of rpoS, assessed with an rpoS-lacZ transcriptional fusion, was positively influenced by GacS and GacA, an effect that was most evident at the transition between exponential growth and stationary phase. Mutations in gacS and gacA compromised the capacity of stationary-phase cells of Pf-5 to survive exposure to oxidative stress. The results of this study provide evidence for the predominant roles of GacS and GacA in the regulatory cascade controlling stress response and antifungal metabolite

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