The rcsA gene of Klebsiella pneumoniae O1:K20 is involved in expression of the serotype-specific K (capsular) antigen.

In Klebsiella pneumoniae, the ability to synthesize large amounts of capsular polysaccharide is an important correlate of virulence. We report the cloning of rcsA from K. pneumoniae serotype O1:K20 and demonstrate that rcsA is involved in the expression of the K antigen capsule. We have determined the nucleotide sequence for the rcsA gene from K. pneumoniae K20 and shown it to be identical to the sequence reported previously for rcsA from strain K21 (Allen et al., J. Gen. Microbiol. 133:331-340, 1987). Southern hybridization results indicate that this gene is widely distributed among different Klebsiella K serotypes. When cloned into Escherichia coli K-12, the K. pneumoniae rcsA gene caused a mucoid phenotype, resulting from the activation of colanic acid synthesis. Activation of colanic acid synthesis was not dependent on growth at low temperatures (less than or equal to 30 degrees C). The K. pneumoniae rcsA gene complemented E. coli K-12 rcsA mutations but could not complement defects in rcsB, suggesting that RcsA may be functionally homologous in these bacteria. The cloned rcsA gene also complemented a defect in nonmucoid strain K20 derivatives that normally produced only trace amounts of K20 antigen and were unable to assemble a wild-type capsular structure. Mutants that were K20-deficient were not complemented. The K antigen capsule of K. pneumoniae therefore joins a growing list of polysaccharide-synthetic systems in which "RcsA-like" proteins are involved.