Sequence of galR gene indicates a common evolutionary origin of lac and gal repressor in Escherichia coli

The nucleotide sequence of the galR gene of Escherichia coli, which codes for galactose repressor, has been determined. The subunits of gal repressor are predicted to consist of 343 residues, including the NH2-terminal methionine. Twenty-six of the predicted NH2-terminal 55 residues of gal repressor are identical to the NH2-terminal residues of lac repressor. Additional homologies appear between residues 165 and 200, between residues 235 and 255, and around residue 325.