The interaction of the two transcriptional regulators RcsA and RcsB with a specific operator is a common mechanism in the activation of capsule biosynthesis in enteric bacteria. We describe RcsAB binding sites in the wza promoter of the operon for colanic acid biosynthesis in Escherichia coli K-12, in the galF promoter of the operon for K2 antigen biosynthesis in Klebsiella pneumoniae, and in the tviA (vipR) promoter of the operon for Vi antigen biosynthesis in Salmonella typhi. We further show the interaction of RcsAB with the rcsA promoters of various species, indicating that rcsA autoregulation also depends on the presence of both proteins. The compilation of all identified RcsAB binding sites revealed the conserved core sequence TaAGaatatTCctA, which we propose to be termed RcsAB box. The RcsAB box is also part of Bordetella pertussis BvgA binding sites and may represent a more distributed recognition motif within the LuxR superfamily of transcriptional regulators. The RcsAB box is essential for the induction of Rcs-regulated promoters. Site-specific mutations of conserved nucleotides in the RcsAB boxes of the E. coli wza and rcsA promoters resulted in an exopolysaccharide-negative phenotype and in the reduction of reporter gene expression.