A model of the lac repressor–operator complex based on physical and genetic data

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Abstract
Computer graphics were used to build a molecular model of the complex of Lac repressor and lac operator. The model is based (a) on the NMR data of the Kaptein group [Boelens, R., Lamerichs, R. M. J. N., Rullmann, J. A. C., van Boom, J. H. & Kaptein, R. (1988) Protein Sequence Data Anal. 1, 487–498] and (b) on our genetic and biochemical data including specificity changes [Lehming, N., Sartorius, J., Kisters‐Woike, B., von Wilcken‐Bergmann, B. & Müller‐Hill, B. (1990) EMBO J. 9, 615–621]. Effects of amino acid exchanges in the recognition helix could be predicted by the model and were subsequently tested and confirmed by genetic experiments. Comparison of the modelled lac complex with the known crystallographic structures of several helix‐turn‐helix DNA complexes reveals striking similarities and suggests rules which govern the recognition between particular amino acid side chains and particular base pairs in these systems.