Transition of Dephospho-DctD to the Transcriptionally Active State via Interaction with Dephospho-IIA(Glc)

Exopolysaccharides (EPSs), biofilm-maturing components of Vibrio vulnificus, are abundantly produced when the expression of two major EPS gene clusters is activated by an enhancer-binding transcription factor, DctD(2), whose expression and phosphorylation are induced by dicarboxylic acids. Surprisingly, when glucose was supplied to V. vulnificus, similar levels of expression of these clusters occurred, even in the absence of dicarboxylic acids. This glucose-dependent activation was also mediated by DctD(2), whose expression was sequentially activated by the transcription regulator NtrC. Most DctD(2) in cells grown without dicarboxylic acids was present in a dephosphorylated state, known as the transcriptionally inactive form. However, in the presence of glucose, a dephosphorylated component of the glucose-specific phosphotransferase system, d-IIA(Glc), interacted with dephosphorylated DctD(2) (d-DctD(2)). While d-DctD(2) did not show any affinity to a DNA fragment containing the DctD-binding sequences, the complex of d-DctD(2) and d-IIA(Glc) exhibited specific and efficient DNA binding, similar to the phosphorylated DctD(2). The d-DctD(2)-mediated activation of the EPS gene clusters’ expression was not fully achieved in cells grown with mannose. Furthermore, the degrees of expression of the clusters under glycerol were less than those under mannose. This was caused by an antagonistic and competitive effect of GlpK, whose expression was increased by glycerol, in forming a complex with d-DctD(2) by d-IIA(Glc). The data demonstrate a novel regulatory pathway for V. vulnificus EPS biosynthesis and biofilm maturation in the presence of glucose, which is mediated by d-DctD(2) through its transition to the transcriptionally active state by interacting with available d-IIA(Glc).