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Model of the 93-base pair loop that forms to repress the transcription of the Escherichia coli lac operon structural genes. The lac repressor tetramer (violet ribbon) binds two operators (red), and the catabolite activator protein (purple ribbon) binds to a separate DNA site (blue). The lac promoter regions are highlighted in green. See page 1247 and the related Perspective on page 1245. [Image created with Biosym Insight II]

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Science. ISSN 0036-8075 (print), 1095-9203 (online)