In Klebsiella pneumoniae the gene products involved in the degradation of the ketose L-sorbose are encoded in the sor operon. It comprises, besides structural genes for uptake and catabolism, a promoter-proximal gene sorC, encoding a protein SorC of Mr 40 kDa, for which no enzymatic function has been detected. All sor genes are coordinately expressed and inducible by L-sorbose. Polar insertions and frameshift mutations in sorC cause a pleiotropic negative effect on the expression of all other sor genes. This defect is complemented in trans by the wild-type sorC+ allele for frameshift mutations, but not for polar insertions. A single promoter for all sor genes, for which SorC is the activator, thus seems to be located in front of sorC. The repressor activity of SorC was demonstrated by complementation of constitutive sorC alleles with a sorC+ allele leading to inducible expression of all sor genes, including sorC, which, as visualized by the use of a series of lacZ fusions, thus autoregulates its expression, both as an activator and a repressor.