CsgD

CsgD is a transcription and response regulator protein referenced to as the master modulator of bacterial biofilm development. In "E. coli" cells, CsgD is tasked with aiding the transition from planktonic cell motility to the stationary phase of biofilm formation, in response to environmental growth factors. A transcription analysis assay (happening "in vitro") illustrated a heightened decrease in CsgD's DNA-binding capacity when phosphorylated at A.A. D59 of the protein's primary sequence. Therefore, in the protein's active form (unphosphorylated), CsgD is capable of carrying out its normal functions of regulating curli proteins (fimbria) and producing ECM polysaccharides (cellulose). Following a promoter-"lacZ" fusion assay of "CsgD binding" to specific target sites on E. coli's genome, two classes of binding targets were identified: "group I genes" and "group II genes". The group I genes, akin to "fliE" and "yhbT", exhibit repressed transcription following their interaction with CsgD, whilst "group II genes", including "yccT" and "adrA", illustrated active functionality. Other group I operons that illustrate repressed transcription include "fliE" and "fliEFGH," for motile flagellum formation. Other group II genes, imperative to the transition towards stationary biofilm development, include "csgBA", encoding for curli fimbriae, and "adrA", encoding for the synthesis of cyclic diguanylate. In this context, c-di-GMP functions as a bacterial secondary messenger, enhancing the production of extracellular cellulose and impeding flagellum production and rotation. 