Repressors

Repressors are regulatory elements that reduce or inhibit gene expression. In genetics, repressors are often proteins that bind to specific DNA sequences, such as operators or silencers, and interfere with transcription initiation or elongation. Repression can also occur when repressor proteins recruit other factors that modify chromatin or DNA structure to a less transcriptionally active state.

In bacteria, repressors commonly regulate operons. The lac repressor LacI binds to operator sites in the lac operon, blocking RNA polymerase and preventing transcription. The presence of an inducer, allolactose, decreases LacI’s affinity for DNA and allows transcription. The tryptophan repressor TrpR binds tryptophan as a corepressor to shut down the trp operon when tryptophan is plentiful. Many bacterial repressors operate through allosteric control, altering DNA-binding in response to small molecules.

In eukaryotes, repression often involves multi-protein complexes that modify chromatin. Repressors such as REST (repressor element-1 silencing transcription factor) and the Groucho/TLE family recruit corepressors (e.g., NCoR/SMRT, Sin3) and histone deacetylases, leading to a compact chromatin state and reduced transcription. Repression can also occur at promoters via competitive binding or through silencers that recruit repressor complexes.

Repressors play essential roles in development, metabolism, and responses to environmental changes. They function alongside activators within gene regulatory networks and can form feedback loops that shape expression patterns. In research and biotechnology, repressor systems are used to construct gene circuits and tune expression levels.