Escherichia coli lactose repressor Isolation of two different homogeneous headpieces and the existence of a hinge region between residues 50 and 60 in the repressor molecule

The lactose repressor controls the expression of the luc genes in E. coli. The specific binding of the repressor to the luc operator DNA segment inhibits the transcription of the polycistronic messenger RNA representing the structural genes of the lac operon (reviewed [ 1,2]). Lac repressor shows not only the base-specific strong binding to operator DNA but also a much weaker interaction with DNA containing non-operator base sequences [3]. There is overwhelming genetic and biochemical evidence indicating that the amino-terminal segments (‘headpieces’) of the tetrameric lac repressor are a necessary requirement for both operator and non-operator DNA binding [3-61. We have recently shown that selective trypsin cleavage of native luc repressor under highly restricted conditions (1 M Tris-HCI, 30% in glycerol, pH 7 S, room temp.) produces a mixture of monomeric ‘headpieces’ accounting for residues l-5 1 and l-59 together with a trypsin resistant tetrameric core [7]. The tetrameric repressor core is devoid of all detectable DNA binding activities [3,5], whereas the headpieces retain the ability to form complexes with non-operator DNA but do not show the high specificity of repressor for operator sequences [8]. The two headpieces can be separated on DNA-cellulose and show different DNA binding properties [8]. In order to facilitate further studies on the interaction of headpieces with DNA we have probed the repressor structure under native conditions with two other proteolytic enzymes in the hope of obtaining homo-