The nucleotide sequence upstream of the Escherichia coli yebG gene presents features similar to those found in SOS system regulatory sites (putative SOS box, -10 and -35 promoter boxes and a ribosome binding site). Operon fusion assays demonstrate now that this region controls transcription in a recA-, lexA-dependent way and that the reporter gene expression is inducible by DNA damage consequent to mitomycin C treatment. Increased expression does not result from an increase in plasmid copy number. These results indicate that yebG is a novel SOS regulon gene. The yebG product is predicted to be a 96 amino acid residue, 10.7 kDa protein whose function is not yet known. Unlike other SOS genes, the construct carrying the yebG regulatory region is not stationary phase inducible.