repressor

A repressor is a molecule that reduces gene expression. In genetics, repressors are often proteins that bind to specific DNA sequences, such as operators or promoter-proximal sites, or that interact with other transcription factors to block transcription. Repression is a form of negative regulation in which the presence or activity of the repressor lowers the rate at which a gene is transcribed.

In bacteria, repressors control operons. The LacI repressor binds to the lac operator and prevents RNA polymerase from initiating transcription. When an inducer such as allolactose binds LacI, the protein changes shape and releases from DNA, allowing transcription. The TrpR repressor binds the operator only when complexed with tryptophan, suppressing the tryptophan operon.

Repressor function can involve direct DNA blocking or recruitment of additional factors. Many repressors function as dimers and recognize palindromic DNA motifs. They may be regulated allosterically by small molecules, light, or other signals, so that repression is reversible. In larger regulatory networks, repressors often cooperate with activators and corepressors to fine-tune gene expression.

In eukaryotes, repressors can silence genes by recruiting corepressors and chromatin-modifying enzymes, such as histone deacetylases, or by promoting nucleosome compaction. They may bind silencers or interfere with transcriptional activators. Repression is a key component of developmental programs and environmental responses, and it complements activation to create complex gene expression patterns.

Examples include LacI, TrpR, and LexA, which regulate metabolism, amino acid biosynthesis, and the SOS response, respectively. The concept of repression extends beyond prokaryotes to many eukaryotic transcriptional networks, where repressors contribute to gene silencing, dosage compensation, and epigenetic control.