repressor

Repressor signals typically work by permiting proteins to bind on top of the promoter sequence, physically blocking promoters. Within bacterial operons, regulator genes code for repressor proteins that bind to the operator, thus obstructing the promoters (transcription) of the structural genes. The regulator genes need not be adjacent to other genes in the operon. If the repressor protein is removed, transcription may occur. Both repression and induction are examples of negative control since the repressor proteins turn off transcription.

Repressors and other regulatory proteins probably search for target sequences by binding to DNA and scanning its major groove. Most regulatory proteins recognize DNA because they can slide along it while searching for a specific target sequence. Such a search would be much slower for a protein that cannot bind DNA. [s]

Nrd genes