Identification of a global repressor gene, rsmA, of Erwinia carotovora 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), polygalacturonase (Peh), cellulase (Cel), and protease (Prt) is activated by the cell density (quorum)-sensing signal, N-(3-oxohexanoyl)-L-homoserine lactone (HSL); plant signals; and aep genes during postexponential 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, rsmA (rsm, repressor of secondary metabolites). Nucleotide sequencing, transcript assays, and protein analysis established that a 183-bp open reading frame encodes the 6.8-kDa RsmA. rsmA has extensive homology with the csrA gene of Escherichia coli, which specifies a negative regulator of carbon storage. Moreover, the suppression of glycogen synthesis 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 other enterobacteria such as Enterobacter aerogenes, E. coli, Salmonella typhimurium, Shigella flexneri, Serratia marcescens, and Yersinia pseudotuberculosis. rsmA suppresses production of Pel, Peh, Cel, and Prt, plant pathogenicity, and synthesis of HSL in E. carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, E. carotovora subsp. carotovora, and E. chrysanthemi. In the E. carotovora subsp. carotovora 71, rsmA reduces the levels of transcripts of hslI, a luxI homolog required for HSL biosynthesis. This specific effect and the previous finding that HSL is required for extracellular enzyme production and pathogenicity in soft-rotting Erwinia spp. support the hypothesis that rsmA controls these traits by modulating the levels of the cell density (quorum)-sensing signal.