To cope with free-floating and surface-adhesion growth,the planktonic and biofilm phenotypes constitute the integral bacterial life.Biofilms are bacterial exopolymer-enclosed communities.They are mainly made up from protein polymers such as curli frimbriae and extracellular polysaccharides.Switching between planktonic and biofilm mode of growth is critical in bacterial development and must be strictly regulated.Biofilm master regulator CsgD switches planktonic growth to biofilm formation by activating synthesis of curli fimbriae and cellulose in Enterobacteriaceae.CsgD consists of C-terminal DNA binding domain and N-terminal receiver domain that provokes the upstream signal transduction to further modulate CsgD activity.We provide the crystal structure of Salmonella Typhimurium CsgD receiver domain,which assembles into an oligomerization state with two non-crystallographic dimerization interfaces.Mutagenesis study indicated that both dimerization interfaces are crucial for CsgD activity.Conserved phosphorylation site D59 is directly involved in stabilization of dimerization interface Ⅰ as the constitutive phosphor-mimetic mutant D59E reduces CsgD activity.Dimerization interface Ⅱ is nearly identical to the c-di-GMP driven dimerization interface in Vibrio cholerae VpsT,although CsgD binds neither c-di-GMP nor c-di-AMP.Our studies provide the structural basis for the regulation of CsgD via oligomerization and phosphorylation of its receiver domain,thus shedding new insights into mechanistic diversity of transcriptional regulators.