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| Modern Genetic Analysis Chapter 14. Regulation of Gene Transcription Regulation of Transcription Factors Figure 14-33. View of the Lac repressorDNA complex, as determined by X-ray crystallography. Here the repressor tetramer is shown binding to two operators. Each dimer binds to one operator. The operators are 21 base pairs long here and are shown in dark and light blue. The monomers are colored green, pink, red, and yellow. Note how the amino-terminal headpiece on one monomer (pink) crosses the other monomer (green) to bind the first part of the operator, whereas the second monomer headpiece (green) crosses over to bind the second part of the operator. Thus, the recognition helix from each of two subunits binds to the consecutive parts of the same operator. (Illustration supplied by M. Lewis, Geoffrey Chang, N. C. Horton, M. A. Kercher, H. C. Pace, M. A. Schumacher, R. G. Brennan, and P. Lu, University of Pennsylvania.)

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