Characterization of salA, syrF, and syrG Regulatory Networks Involved in Plant Pathogenesis by Pseudomonas syringae pv. syringae B728a
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2014-02-06
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Abstract
Pseudomonas syringae pv. syringae B728a, causal agent of brown spot on bean, is an economically important plant pathogen that utilizes extracellular signaling to initiate a lifestyle change from an epiphyte to a pathogen. LuxR regulatory proteins play an important role in the transcriptional regulation of a variety of biological processes involving two-component signaling, quorum sensing, and secondary metabolism. Analysis of the B728a genome identified 24 LuxR-like proteins, three of which are salA, syrF, and syrG located adjacent to the syringomycin gene cluster. All three proteins exhibit domain architecture that placed these LuxR-like proteins into a subfamily of LuxR’s associated with regulation of secondary metabolism in Pss B728a. The transcriptional start sites of salA, syrG, and syrF were located 63, 235, and 498 bp upstream of the start codons, respectively, using primer extension analysis. The predicted -10/-35 promoter region of syrF and syrG was confirmed using site-directed mutagenesis and GFP reporters that showed there were conserved promoter sequences observed around the -35 promoter region. It has been established that SalA binds to the promoter of syrF, therefore these conserved promoter sequences serve as the putative binding site for SalA. Deletion mutants of salA, syrF, and syrG failed to produce syringomycin and displayed reduction of virulence on bean. QRT-PCR analysis results revealed that both syrG and syrF are highly expressed in the apoplast indicating that they encode important transcriptional regulators of genes critical to the plant-pathogen interaction. Additionally, this report showed that syrG and syrF are important transcriptional regulators of syringomycin biosynthesis genes, but are not involved in the regulation of virulence genes that reside outside of the syr-syp gene cluster. Overexpression analysis and GFP reporters identified SyrG as an upstream transcriptional activator of syrF, where both SyrG and SyrF activate promoters of syringomycin biosynthesis genes. This study demonstrates that the interaction between SalA, SyrG, and SyrF for the regulation of syringomycin is complex requiring further investigation.
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Pseudomonas syringae