The uxuAB operon is composed of two genes coding for enzymes involved in hexuronate degradation. This operon is negatively controlled by the uxuR and exuR regulatory gene products. Starting from uxu hybrid plasmids, a 300 bp Sau3A restriction fragment was isolated and shown to contain the entire uxu regulatory region using in vitro gene fusions that brought lac gene expression under the control of the transcriptional and translational signals of the uxuA gene. The nucleotide sequence of this fragment was established. The start of the uxu mRNA was localized by S1 mapping experiments (the presence of a CAP binding site upstream of the promoter was shown by DNAse I footprinting and in vitro titration). The N-terminal amino acid sequence of the purified uxuA-lacZ gene product allowed the mapping of the uxuA initiation codon at 115 bp from the start of transcription.