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The Response Regulator ExpM Is Essential for the Virulence of Erwinia carotovora subsp. carotovora and Acts Negatively on the Sigma Factor RpoS (σS)

July 1999 , Volume 12 , Number  7
Pages  575 - 584

Robert A. Andersson , 1 E. Tapio Palva , 2 and Minna Pirhonen 1

1Department of Plant Biology, Uppsala Genetic Center, Swedish University of Agricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden; 2Department of Biosciences, Division of Genetics, Box 56, FIN-00014 University of Helsinki, Finland

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Accepted 6 April 1999.

The main virulence factors of Erwinia carotovora subsp. carotovora, the secreted, extracellular cell-wall-degrading enzymes, are controlled by several regulatory mechanisms. We have isolated transposon mutants with reduced virulence on tobacco. One of these mutants, with a mutation in a gene designated expM, was characterized in this study. This mutant produces slightly reduced amounts of extracellular enzymes in vitro and the secretion of the enzymes is also affected. The expM wild-type allele was cloned together with an upstream gene, designated expL, that has an unknown function. The expM gene was sequenced and found to encode a protein with similarity to the RssB/SprE protein of Escherichia coli and the MviA protein of Salmonella typhimurium. These proteins belong to a new type of two-component response regulators that negatively regulate the stability of the Sigma factor RpoS (σS) at the protein level. The results of this study suggest that ExpM has a similar function in E. carotovora subsp. carotovora. We also provide evidence that the overproduction of RpoS in the expM mutant is an important factor for the reduced virulence phenotype and that it partly causes the observed phenotype seen in vitro. However, an expM/rpoS double mutant is still affected in secretion of extracellular enzymes, suggesting that ExpM in addition to RpoS also acts on other targets.

Additional keywords: sensory kinase.

© 1999 The American Phytopathological Society