Recognition of the lac operator by the lac repressor involves specific interactions between residues in the repressor's recognition helix and bases in the DNA major groove. Tyr17 and Gln18, at positions 1 and 2 in the lac repressor recognition helix, can be exchanged for other amino acids to generate mutant repressors that display altered specificity. We have solved the solution structure of a protein-DNA complex of an altered-specificity mutant lac headpiece in which Tyr17 and Gln18 were exchanged for valine and alanine, respectively, as found in the recognition helix of the gal repressor. As previously described by Lehming et al. (EMBO J. 1987, 6, 3145-3153), this altered-specificity mutant of the lac repressor recognizes a variant lac operator that is similar to the gal operator Oe. The mutant lac headpiece showed the predicted specificity and is also able to mimic the gal repressor by recognizing and bending the natural gal operator Oe. These structural data show that, while most of the anchoring points that help the lac headpiece to assemble on the lac operator were preserved, a different network of protein-DNA interactions connecting Ala17 and Val18 to bases in the DNA major groove drives the specificity towards the altered operator.