Complex Regulatory Pathways Connect Vfm Quorum-sensing and Cyclic-di-GMP Signaling to Bacterial Virulence in the Phytopathogen Dickeya Dadantii 3937

Abstract

The bacterial second messenger Bis-(3’-5’)-cyclic dimeric guanosine monophosphate (c-di-GMP) regulates multiple cellular behaviors in most bacteria. Bacterial c-di-GMP signaling involves enzymes that synthesize and degrade c-di-GMP, c-di-GMP binding effectors, and targets that are acted upon by the effectors. So far, the c-di-GMP signaling pathways have been understudied. In this work, we explore the c-di-GMP signaling network further in the phytopathogen Dickeya dadantii 3937. In Chapter 2, we identified VfmE as a c-di-GMP binding transcriptional activator that represses pectate lyase production under a high c-di-GMP condition. VfmE was found to bind c-di-GMP in vitro via the RxxxR motif, similar to PilZ domain containing proteins. Distinct differences were observed in pectate lyase (Pel) production when a c-di-GMP insensitive mutant of VfmE was expressed in the cell under a high c-di-GMP condition. We also found that VfmE involves SlyA in the regulation of Pel, thus uncovering a complex regulatory network involving Vfm quorum-sensing, c-di-GMP signaling, and pectate lyase production. In chapter 3, we studied the regulatory mechanism of type III secretion system (T3SS) gene expression by VfmE and the incorporation of c-di-GMP in the regulatory pathway. We demonstrated that VfmE positively regulates the expression of T3SS needle component hrpA at the transcriptional level via HrpL; the master regulator of T3SS. A c-di-GMP insensitive mutant of VfmE lacked the c-di-GMP related phenotype in hrpA transcription. Our study showed the multitiered regulation of hrp genes by VfmE and a high c-di-GMP level caused by deletion of ecpC.