Mutations affecting the quaternary structure of the lac repressor.

A new class of point mutations in the lacI gene of Escherichia coli is described which results in a lac repressor that is unable to form its normal tetrameric structure. Many of these mutations result in amino acid substitutions in the neighborhood of the tyrosine at residue 269. An amber mutation derived from the codon normally specifying tyrosine 269 has been suppressed with Sul, which inserts serine in response to the amber codon. The resulting altered lac repressor is also unable to form its normal tetrameric structure and can no longer bind to the lac operator. Two missense mutations which map in the part of the gene specifying amino acid residues 210 to 216 also produce repressors unable to form tetramers.