Sequence of the ebgR gene of Escherichia coli: evidence that the EBG and LAC operons are descended from a common ancestor.

The sequence of ebgR, the gene that encodes the EBG repressor, was determined. There is 44% DNA sequence identity between ebgR and lacI, the gene that encodes the LAC repressor. There is also 25% identity between the amino acid sequence of lacI and the deduced amino acid sequence of ebgR. The sequence of 596 bp distal to ebgA, the structural gene for EBG beta-galactosidase, was also determined. Within that region there were two sequences, 74 and 100 bp long, that showed 46% and 50% identity, respectively, to sequences in the first 600 bp of lacY, the structural gene for the lactose permease. The organization and direction of transcription of the repressor and structural genes of the two operons are identical. Taken together with the homology between ebgA and lacZ (as demonstrated in the companion article in this issue), this provides strong evidence that the EBG and LAC operons are descended from a common ancestor. The map position of these two operons supports the notion that these operons diverged following a genome duplication event in an ancestor of Escherichia coli.