Escherichia coli produces two distinct types of capsular polysaccharide (designated groups I and II), which are distinguished by chemical, physical, and genetic characteristics. The K30 capsular antigen is a member of the group I, or heat-stable, capsules. We have cloned rcsA from E. coli O9:K30 and determined the nucleotide sequence. The rcsAK30 sequence is virtually identical to the rcsAK-12 sequence (V. Stout, A. Torres-Cabassa, M. R. Maurizi, D. Gutnick, and S. Gottesman, J. Bacteriol. 173:1738-1747, 1991). RcsAK-12 is a transcriptional activator involved in expression of the extracellular polysaccharide colanic acid in E. coli K-12. rcsAK30 complemented an rcsAK-12 mutation and activated colanic acid synthesis in E. coli K-12 strains. However, in E. coli K30, increasing the levels of RcsA by introducing multicopy rcsAK30 or a Lon mutation resulted in elevated synthesis of the K30 capsular polysaccharide; no colanic acid was detected. E. coli K-12 strains in which the chromosomal his region was replaced by that from E. coli K30 were able to synthesize K30 capsular polysaccharide. These K-12/K30 hybrid strains did not produce colanic acid, suggesting that the genes for synthesis of colanic acid and the K30 capsular polysaccharide may be allelic. rcsA sequences were also detected in the group II strains E. coli K1 and K5. Introduction of rcsAK30 into group II strains resulted in activation of colanic acid biosynthesis rather than the group II capsule. Given the role of RcsA in other members of the family Enterobacteriaceae, our results provide further evidence that this protein may be a relatively widespread regulatory component for the synthesis of enterobacterial extracellular polysaccharides.