Question

How are the actions of lac repressor and trp repressor similar and how are they different with regard to their binding to operator sites, their effects on transcription, and the influences of small effector molecules?

Short answer

Both lac and trp repressors bind to their respective operator sites and prevent transcription under certain conditions. However, lac repressor binds when lactose is absent while trp repressor binds when tryptophan is abundant. Small effector molecules like allolactose and tryptophan influence the binding dynamics of these repressors respectively.

Step-by-step solution

Lac Repressor and Operator Site Binding

The lac repressor protein binds to the lac operator site when lactose is not present in the cell, preventing the transcription of the lac operon.

Trp Repressor and Operator Site Binding

The trp repressor protein, on the other hand, binds to the trp operator site when tryptophan, an amino acid, is abundant in the cell, preventing the transcription of the trp operon.

Lac Repressor and Transcription

In terms of effects on transcription, when lactose is present in the cell, it binds to the lac repressor protein, causing the protein to detach from the operator site, enabling the lac operon's transcription.

Trp Repressor and Transcription

In the case of the trp repressor, when tryptophan levels in the cell are low, the trp repressor detaches from the operator site, allowing the transcription of the trp operon.

Lac Repressor and Small Effector Molecules

Small effector molecules, specifically an isomer of lactose called allolactose, influences the lac repressor by binding to it when lactose is present, causing it to change shape and consequently detach from the operator site.

Trp Repressor and Small Effector Molecules

In the case of trp repressor, tryptophan itself acts as a small effector molecule that, when abundant, binds to the trp repressor protein causing it to change shape and bind to the operator site, preventing transcription.