Structural analysis of the umu operon required for inducible mutagenesis in Escherichia coli

The nucleotide sequence of a 2.8-kilobase fragment of the Escherichia coli chromosome containing the umuDC genes has been determined. The DNA sequence specifies two open reading frames of 417 and 1266 nucleotides encoding proteins with calculated molecular weights of 15,063 and 47,677, respectively. From these and the previous results of genetic and biochemical studies on the cloned genes, we conclude that the former is the umuD and the latter is the umuC gene. In vitro transcription of the regulatory region of the umu operon revealed that (i) upstream of the coding region there is a promoter-operator complex having the consensus sequence, CTGTATATAAAAACAG, of an SOS box, (ii) transcription of the umu operon begins at an adenine residue in the SOS box, and (iii) the LexA protein binds to the operator region, with an equilibrium dissociation constant (Kd) of 0.2, thereby repressing the transcription of the umuDC genes. These results indicate that the expression of the umu operon is under the coordinated control of the recA lexA gene products.