Role of the Biofilm Master Regulator CsgD in Cross-Regulation between Biofilm Formation and Flagellar Synthesis

CsgD, the master regulator of biofilm formation, activates the synthesis of curli fimbriae and extracellular polysaccharides inEscherichia coli . To obtain insights into its regulatory role, we have identified a total of 20 novel regulation target genes on theE. coli genome by using chromatin immunoprecipitation (ChIP)-on-chip analysis with a high-density DNA microarray. By DNase I footprinting, the consensus CsgD-binding sequence predicted from a total of 18 target sites was found to include AAAAGNG(N2 )AAAWW. After a promoter-lacZ fusion assay, the CsgD targets were classified into two groups: group I genes, such asfliE andyhbT , are repressed by CsgD, while group II genes, includingyccT andadrA , are activated by CsgD. ThefliE andfliEFGH operons for flagellum formation are directly repressed by CsgD, while CsgD activates theadrA gene, which encodes an enzyme for synthesis of cyclic di-GMP, a bacterial second messenger, which in turn inhibits flagellum production and rotation. Taking these findings together, we propose that the cell motility for planktonic growth is repressed by CsgD, thereby promoting the switch to biofilm formation.