Molecular characterization of global regulatory RNA species that control pathogenicity factors in Erwinia amylovora and Erwinia herbicola pv. gypsophilae

rsmB(Ecc) specifies a nontranslatable RNA regulator that controls exoprotei n production and pathogenicity in soft rot-causing Erwinia carotovora subsp . carotovora. This effect of rsmB(Ecc) RNA is mediated mostly by neutralizi ng the function of RsmA(Ecc), an RNA-binding protein of E. carotovora subsp . carotovora, which acts as a global negative regulator. To determine the o ccurrence of functional homologs of rsmB(Ecc) in non-soft-rot-causing Erwin ia species, we cloned the rsmB genes of E. amylovora (rsmB(Ea)) and E. herb icola pv. gypsophilae (rsmB(Ehg)). We show that rsmB, in E. amylovora posit ively regulates extracellular polysaccharide (EPS) production, motility, an d pathogenicity. In E. herbicola pv. gypsophilae, rsmB(Ehg) elevates the le vels of transcripts of a cytokinin (etz) gene and stimulates the production of EPS and yellow pigment as well as motility, RsmA(Ea) and RsmA(Ehg) have more than 93% identity to RsmA(Ecc) and, like the latter, function as nega tive regulators by affecting the transcript stability of the target gene. T he rsmB genes reverse the negative effects of RsmA(Ea), RsmA(Ehg), and RsmA (Ecc), but the extent of reversal is highest with homologous combinations o f rsm genes. These observations and findings that rsmB(Ea) and rsmB(Ehg) RN A bind RsmA(Ecc) indicate that the rsmB effect is channeled via RsmA. Addit ional support for this conclusion comes from the observation that the rsmB genes are much more effective as positive regulators in a RsmA(+) strain of f. carotovora subsp, carotovora than in its RsmA(-) derivative. E, herbicol a pv. gypsophilae produces a 290-base rsmB transcript that is not subject t o processing. By contrast, E, amylovora produces 430- and 300-base rsmB tra nscripts, the latter presumably derived by processing of the primary transc ript as previously noted with the transcripts of rsmB(Ecc). Southern blot h ybridizations revealed the presence of rsmB homologs in E. carotovora, E. c hrysanthemi, E. amylovora, E. herbicola, E. stewartii and E. rhapontici, as well as in other enterobacteria such as Escherichia coli, Salmonella enter ica serovar Typhimurium, Serratia marcescens, Shigella flexneri, Enterobact er aerogenes, Klebsiella pneumoniae, Yersinia enterocolitica, and Y. pseudo tuberculosis, A comparison of rsmB sequences from several of these enteroba cterial species revealed a highly conserved 34-mer region which is predicte d to play a role in positive regulation by rsmB RNA.