Submitted to: Journal of Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 1999
Publication Date: N/A
Interpretive Summary: We are studying the mechanism by which bacteria are able to cause plant disease. Towards this end, we are analyzing the Pseudomonas syringae pv. syringae (the causative agent of brown spot disease of snap bean - an important field disease in Wisconsin) as a model system. We hope to gain an understanding of the molecular biology of the genes and gene products that are required for the disease process. This manuscript describes the identification and analysis of a new gene, salA, within the gacS and gacA region of B728a. This is the first description of this unique gene. The salA gene regulates lesion formation on bean and production of the toxin, syringomycin. Ultimately, this understanding will provide information of use to breeders and plant molecular biologists that will enable them to develop plant lines resistant to disease organisms through the alteration of specific plant products that are required by the pathogen.
The genes lemA (which we here redesignate gacS) and gacA encode members of a widely conserved two-component regulatory system. In Pseudomonas syringae strain B728a, gacS and gacA are required for lesion formation on bean, as well as for production of protease and the toxin syringomycin. A gene, designated salA, was discovered that restored syringomycin production to a gacS mutant when present on a multiple-copy plasmid. Disruption of chromosomal salA resulted in loss of syringomycin production and lesion formation in laboratory assays. Sequence analysis of salA suggests that it encodes a protein with a DNA-binding motif but without other significant similarity to proteins in current databases. Chromosomal reporter fusions revealed that gacS and gacA positively regulate salA, that salA upregulates its own expression, and that salA positively regulates the expression of a syringomycin biosynthetic gene, syrB. Loss of syringomycin production does not account for the salA mutant's attenuated pathogenicity, since a syrB mutant was found to retain full virulence. The salA gene did not similarly suppress the protease deficient phenotype of gacS mutants, nor were salA mutants affected for protease production. A gacS/gacA dependent homoserine lactone activity as detected by bioassay was also unaffected by disruption of salA. Thus, salA appears to encode a novel regulator that activates the expression of at least two separate genetic subsets of the gacS/gacA regulon, one pathway leading to syringomycin production and the other resulting in plant disease.