lon mutants of Escherichia coli K-12 are defective in an ATP-dependent protease, are UV sensitive, and overproduce the capsular polysaccharide colanic acid. Six structural genes needed for capsular polysaccharide synthesis (cps) are transcriptionally regulated by lon as well as by three other regulatory genes, rcsA, -B, and -C (S. Gottesman, P. Trisler, and A. S. Torres-Cabassa, J. Bacteriol. 162:1111-1119, 1985). We have cloned rcsA, the gene for a positive regulator of capsule synthesis, onto multicopy plasmids and defined the gene by both insertions and deletions. The product of rcsA has been identified as an unstable protein of 27 kilodaltons. RcsA has a half-life of 5 min in lon+ cells and one of 20 min in lon cells. The availability of RcsA is the limiting factor for capsule synthesis; doubling the gene dosage of rcsA+ significantly increases expression of cps genes. Our results are consistent with a model in which the presence of a lon mutation increases the synthesis of capsular polysaccharide via stabilization of RcsA.