GENETIC-EVIDENCE THAT THE GACA GENE ENCODES THE COGNATE RESPONSE REGULATOR FOR THE LEMA SENSOR IN PSEUDOMONAS-SYRINGAE

Mutational analysis of the bean-pathogenic Pseudomonas syringae pv. sy ringae strain B728a has led to the genetic identification of the gacA gene as encoding the response regulator for the unlinked lemA sensor k inase. The analysis of a collection of spontaneous mutants of P. syrin gae pv. syringae suggested that the gacA gene was involved in lesion f ormation and the production of protease and syringomycin. The gacA gen e originally was identified as a regulator of extracellular antibiotic production by Pseudomonas fluorescens, and the predicted GacA protein is a member of the FixJ family of bacterial response regulators. The sequence of the putative B728a GacA protein revealed 92% identity with the P. fluorescens GacA protein. An insertional mutation within the P . syringae pv. syringae gacA gene abrogated lesion formation on beans, production of extracellular protease, and production of the toxin syr ingomycin, the same phenotypes affected by a lemA mutation. DNA sequen ce analysis identified the P. syringae pv, syringae uvrC gene immediat ely downstream of the gacA gene, an arrangement conserved in P. fluore scens and Escherichia coli. The gacA insertional mutant was sensitive to UV, presumably because of polarity on transcription of the downstre am uvrC gene. Southwestern (DNA-protein) analysis revealed that the le mA and gacA genes were required for the full expression of a DNA bindi ng activity.