A NEWLY IDENTIFIED REGULATOR IS REQUIRED FOR VIRULENCE AND TOXIN PRODUCTION IN PSEUDOMONAS-SYRINGAE

The genes lemA (which we here redesignate gacS) and gacA encode member s of a widely conserved two-component regulatory system. In Pseudomona s syringae strain B728a, gacS and gacA are required for lesion formati on on bean, as well as for the production of protease and the toxin sy ringomycin. A gene, designated salA, was discovered that restored syri ngomycin production to a gacS mutant when present on a multiple-copy p lasmid, Disruption of chromosomal salA resulted in loss of syringomyci n production and lesion formation in laboratory assays. Sequence analy sis of salA suggests that it encodes a protein with a DNA-binding moti f but without other significant similarity to proteins in current data bases. Chromosomal reporter fusions revealed that gacS and gacA positi vely regulate salA, that salA upregulates its own expression and that salA positively regulates the expression of a syringomycin biosyntheti c gene, syrB, Loss of syringomycin production does not account for the salA mutant's attenuated pathogenicity, as a syrB mutant was found to retain full virulence. The salA gene did not similarly suppress the p rotease deficient phenotype of gacS mutants, nor were salA mutants aff ected for protease production. A gacS/gacA-dependent homoserine lacton e activity as detected by bioassay was also unaffected by the disrupti on of salA, Thus, salA appears to encode a novel regulator that activa tes the expression of at least two separate genetic subsets of the gac S/gacA regulon, one pathway leading to syringomycin production and the other resulting in plant disease.