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.

The lac repressor, also called LacI, is the regulatory protein that reduces transcription of the lac operon by binding a lac operator in Escherichia coli. Binding of a small-molecule inducer changes the repressor's regulatory state through allostery. Allolactose connects this switch with lactose metabolism, while IPTG supplies an experimentally useful inducer. The lac repressor is distinct from the operator DNA and from the RNA polymerase whose transcriptional output is controlled.

Walter Gilbert and Benno Müller-Hill's 1966 isolation experiment gave the proposed regulator a biochemical measurement. Their equilibrium dialysis assay used radioactive IPTG: small free inducer could cross the membrane, while the macromolecular binding component was retained. Extra total labelled inducer on the protein side reported binding. The assay deliberately used inducer recognition rather than assuming that the exact mechanism of transcriptional repression was already established.^[ref]

An original one-site binding model explains that readout. If the free inducer concentration is $I$, the concentration of independent binding sites is $B_T$ and the dissociation constant is $K_I$, the bound concentration isAt equilibrium dialysis with matched solvent conditions, the protein side contains free plus bound inducer, $I+B$, while the opposite side contains free inducer $I$. Choosing $B_T=2\mu \mathrm{M}$ and $I=K_I=1\mu \mathrm{M}$ gives $B=1\mu \mathrm{M}$ and total concentrations $2$ versus $1\mu \mathrm{M}$. Equality of the free concentration does not require equality of the total concentration. The numbers are illustrative; the native lac repressor has a more detailed oligomeric and allosteric structure.

A separate one-operator model connects binding to expression. If a concentration $R$ of binding-competent repressor occupies the lac operator with probability $\theta =R/(K_O+R)$, and occupied DNA produces no transcripts in this model, thenReducing $R/K_O$ from $9$ to $1$ raises the model's transcription rate from $0.1r_0$ to $0.5r_0$. It does not follow that every measured cellular response obeys this one-site expression: operator arrangement, additional regulators and the later stages of gene expression affect the output. Measuring inducer binding and measuring repression therefore test related but different parts of the mechanism.

The repressor controlling the lac operon. Operator binding represses transcription, while inducer binding shifts the conformational equilibrium towards lower operator occupancy.