Benjamin J. Koestler,2
Christopher M. Waters,2
George W. Sundin,3
William Hutchins,1 and
1Department of Biological Sciences, University of Wisconsin-Milwaukee 53211, U.S.A.; 2Department of Microbiology and Molecular Genetics, and 3Department of Plant, Soil, and Microbial Sciences, Michigan State University, East Lansing 48824, U.S.A.; 4Department of Plant Pathology, College of Agronomy & Biotechnology, China Agricultural University, Beijing 100193, China
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Accepted 21 June 2014.
The bacterial soft rot pathogen Dickeya dadantii utilizes the type III secretion system (T3SS) to suppress host defense responses, and secretes pectate lyase (Pel) to disintegrate the plant cell wall. A transposon mutagenesis fluorescence-activated cell sorting screen was used to identify mutants with altered promoter activities of the T3SS pilus gene hrpA. Several insertion mutations, resulting in changes in hrpA expression, were mapped to a new locus, opgGH, which encodes the gene cluster responsible for osmoregulated periplasmic glucan (OPG) synthesis proteins. Our data showed that OPG was involved in T3SS and Pel regulation by altering the expression of the regulatory small RNA RsmB. Through genome searching, the mechanism of two novel regulatory components, the RcsCD-RcsB phosphorelay and CsrD on OPG and the rsmB gene, was further investigated. The Rcs phosphorelay and OPG inversely regulated rsmB at transcriptional and post-transcriptional levels, respectively. CsrD exhibited dual functionality in T3SS and Pel regulation by manipulating levels of RsmB RNA and cyclic diguanylate monophosphate (c-di-GMP). CsrD positively regulated the promoter activity of the rsmB gene but negatively controlled RsmB RNA at the post-transcriptional level via OpgGH. In addition, CsrD contains both GGDEF and EAL domains but acted as a c-di-GMP phosphodiesterase. When the expression of the csrD gene was induced, CsrD regulated T3SS expression and Pel production through controlling intracellular c-di-GMP levels.
© 2014 The American Phytopathological Society