Lac repressor
What is the lac repressor?What is the lac repressor?
Repressors are proteins that inhibit the expression of genes; that is, they inhibit the transcription of messenger RNA from their target genes. The lac repressor controls the expression of bacterial enzymes involved in the metabolism of of the sugar lactose.
For a general introduction to the lac repressor, please see David Goodsell's Introduction to the lac repressor in his series Molecule of the Month, and the article in Wikipedia on the lac repressor.