 Transcriptional repressor

A transcriptional repressor is a regulatory protein that lowers transcription of a specified target under stated conditions. Repression can involve blocking access or activity of the transcription machinery or recruiting additional regulatory interactions. A reduction in final protein abundance alone does not identify transcriptional repression: RNA degradation, translation or protein turnover could also change that abundance.

LacI supplies a concrete example. It binds a lac operator associated with the lac operon, reducing transcription; inducer binding changes the regulator's behavior. The 1966 paper by Walter Gilbert and Benno Müller-Hill isolated the repressor and framed operator recognition as the physical mechanism connecting a regulatory product to negative control.^[ref]

A transparent one-site model separates binding from output. Let free repressor concentration be $R$, its operator dissociation constant $K_d$, and occupancy $\theta =R/(K_d+R)$. If transcription occurs at rate $r_0$ only when the site is unoccupied, thenFor chosen $K_d=10\mathrm{nM}$, raising $R$ from $10$ to $90\mathrm{nM}$ lowers this model's rate from $r_0/2$ to $r_0/10$. This is an illustrative equilibrium gate, not a complete quantitative model of the lac system: multiple sites, looping, cooperativity and non-equilibrium kinetics can alter the response.

An experimental test compares an intact operator with a binding-site variant while measuring both regulator binding and an appropriate transcription readout. A reporter signal additionally depends on later molecular steps, so direct RNA measurements can test whether the change occurs at transcription. A transcription corepressor can assist repression through recruited interactions without being equivalent to the DNA-recognizing regulator. Specifying the molecular target and the readout keeps these mechanisms distinct.