Complex of lac repressor headpiece with a 14 base-pair lac operator fragment studied by two-dimensional nuclear magnetic resonance

Mechanism of Lac repressor switch‐off: orientation of the Lac repressor DNA‐binding domain is reversed upon inducer binding

“…This structure contains the specific DNA-binding 'helix-turn-helix' motif, found in other DNA-binding proteins by X-ray studies [6] and predicted for the lac repressor [7]. The orientation of the lac repressor recognition helix relative to the operator was shown to be opposite [5,8] to that established for all other helix-turnhelix proteins for which the structure was determined by X-ray analysis, such as phage A cro protein [9].…”

“…Similarly, mainly N-termini were cross-linked at the presence of inducer to the lac operator and the attachement was located at the operator boundaries. NMR studies showed that the lac repressor N-terminus resides close to the lac operator axis of symmetry, but does not make the direct contact with the operator [5,[18][19][20] (Fig. 4A).…”

“…Within the specific repressoroperator complex this helix has the 'specific' lac orientation [5,8,19,20,22,23]. This orientation arises upon the operator's influence.…”

“…The three-dimensional structure of the lac repressor headpiece has been solved by NMR [4,5]. This structure contains the specific DNA-binding 'helix-turn-helix' motif, found in other DNA-binding proteins by X-ray studies [6] and predicted for the lac repressor [7].…”

Photo‐CIDNP study of the interaction between lac repressor headpiece and lac operator DNA

“…Using 2D NMR methods, the spatial structure of this domain (HP51) was solved [3,4]. Furthermore, based on the observation of NOES between protein and DNA, a preliminary model of the complex of lac headpiece with a 14 bp lac operator was proposed [5]. The most striking feature of this model was that the second a-helix of the helix-turn-helix motif of lac headpiece binds in the major groove of DNA in an orientation opposite to that found for other repressors [6].…”