RcsB and RcsC: a two-component regulator of capsule synthesis in Escherichia coli.

Colanic acid capsule synthesis in Escherichia coli K-12 is regulated by RcsB and RcsC. The amino acid sequences of these two proteins, deduced from the nucleotide sequence reported here, demonstrate their homology to environmentally responsive two-component regulators that have been reported in both gram-positive and gram-negative bacteria. In our model, RcsC acts as the sensor and RcsB acts as the receiver or effector to stimulate capsule synthesis from cps genes. In addition, RcsC shows limited homology to the other effectors in its C terminus. Fusions of rcsC to phoA that resulted in PhoA+ strains demonstrated that RcsC is a transmembrane protein with a periplasmic N-terminal domain and cytoplasmic C-terminal domain. Additional control of this regulatory network is provided by the dependence on the alternate sigma factor, RpoN, for the synthesis of RcsB. The rcsB and rcsC genes, which are oriented convergently with their stop codons 196 base pairs apart, are separated by a long direct repeat including two repetitive extragenic palindromic sequences.