IDENTIFICATION OF A GLOBAL REPRESSOR GENE, RSMA, OF ERWINIA-CAVOTAVORA SUBSP CAROTOVORA THAT CONTROLS EXTRACELLULAR ENZYMES, N-(3-OXOHEXANOYL)-L-HOMOSERINE LACTONE, AND PATHOGENICITY IN SOFT-ROTTING ERWINIA SPP

The production of extracellular enzymes such as pectate lyase (Pel), p olygalacturonase (Peh), cellulase (Cel), and protease (Prt) is activat ed by the cell density (quorum)-sensing signal, N-(3-oxohexanoyl)-L-ho moserine lactone (HSL); plant signals; and aep genes during postexpone ntial growth of Erwinia carotovora subsp. carotovora 71. Studies with mutants of E. carotovora subsp. carotovora 71 derepressed in exoenzyme production led to the identification of a negative regulator gene, rs mA (rsm, repressor of secondary metabolites). Nucleotide sequencing, t ranscript assays, and protein analysis established that a 183-bp open reading frame encodes the 6.8-kDa RsmA. rsmA has extensive homology wi th the csrA gene of Escherichia coli, which specifies a negative regul ator of carbon storage. Moreover, the suppression of glycogen synthesi s in E. coli by rsmA. indicates that the Erwinia gene is functionally similar to csrA. Southern hybridizations revealed the presence of rsmA homologs in soft-rotting and non-soft-rotting Erwinia spp. and in oth er enterobacteria such as Enterobacter nerogenes, E. coli, Salmonella typhimurium, Shigella flexneri, Serratia marcescens, and Yersinia pseu dotuberculosis. rsmA suppresses production of Pel, Feb, Cel, and Prt, plant pathogenicity, and synthesis of HSL in E. carotovora subsp. atro septica, E. carotovora subsp. betavasculorum, E. carotovora subsp. car otovora, and E. chrysanthemi. In the E. carotovora subsp. carotovora 7 1, rsmA reduces the levels of transcripts of hsII, a luxI homolog requ ired for HSL biosynthesis. This specific effect and the previous findi ng that HSL is required for extracellular enzyme production and pathog enicity in soft-rotting Erwinia spp. support the hypothesis that rsmA controls these traits by modulating the levels of the cell density (qu orum)-sensing signal.