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Journal of Bacteriology, May 2006, p. 3290-3298, Vol. 188, No. 9
0021-9193/06/$08.00+0     doi:10.1128/JB.188.9.3290-3298.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Characterization of the Transcriptional Activators SalA and SyrF, Which Are Required for Syringomycin and Syringopeptin Production by Pseudomonas syringae pv. syringae

Nian Wang,{dagger} Shi-En Lu,{ddagger} Angela R. Records, and Dennis C. Gross*

Department of Plant Pathology and Microbiology, Texas A&M University, College Station, Texas 77843

Received 29 November 2005/ Accepted 13 February 2006

Production of the phytotoxins syringomycin and syringopeptin by Pseudomonas syringae pv. syringae is controlled by the regulatory genes salA and syrF. Analysis with 70-mer oligonucleotide microarrays established that the syr-syp genes responsible for synthesis and secretion of syringomycin and syringopeptin belong to the SyrF regulon. Vector pMEKm12 was successfully used to express both SalA and SyrF proteins fused to a maltose-binding protein (MBP) in Escherichia coli and P. syringae pv. syringae. Both the MBP-SalA and MBP-SyrF fusion proteins were purified by maltose affinity chromatography. Gel shift analysis revealed that the purified MBP-SyrF, but not the MBP-SalA fusion protein, bound to a 262-bp fragment of the syrB1 promoter region containing the syr-syp box. Purified MBP-SalA caused a shift of a 324-bp band containing the putative syrF promoter. Gel filtration analysis and cross-linking experiments indicated that both SalA and SyrF form homodimers in vitro. Overexpression of the N-terminal regions of SalA and SyrF resulted in decreased syringomycin production by strain B301D and reduced levels of ß-glucuronidase activities of the sypA::uidA and syrB1::uidA reporters by 59% to 74%. The effect of SalA on the expression of the syr-syp genes is mediated by SyrF, which activates the syr-syp genes by directly binding to the promoter regions. Both SalA and SyrF resemble other LuxR family proteins in dimerization and interaction with promoter regions of target genes.

* Corresponding author. Mailing address: Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843. Phone: (979) 845-8288. Fax: (979) 845-6483. E-mail: d-gross{at}tamu.edu.

{dagger} Present address: Plant and Microbial Biology Department, University of California, Berkeley, CA 94720.

{ddagger} Present address: Department of Entomology and Plant Pathology, Mississippi State University, Mississippi State, MS 39762.

Journal of Bacteriology, May 2006, p. 3290-3298, Vol. 188, No. 9
0021-9193/06/$08.00+0     doi:10.1128/JB.188.9.3290-3298.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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