Figure 1.

Rationale for the repressed transactivtor two-hybrid system. (A) Representation of the GAL1 gene in glucose-grown cells, indicating the binding sites for GAL4 (UAS), and MIG₁ (URS). MIG₁ recruits the general repressors SSN6 and TUP1. The N-terminal RD of TUP1 is indicated in black. (B) Activation of a GAL1-URA3 reporter gene by a GAL4 DBD-activator fusion protein (activator bait) in ura3⁻ yeast gives a URA⁺ phenotype, and causes sensitivity to 5-FOA. (C) Interaction between a TUP1 RD fusion (TUP1-RD prey) with the activator bait causes repression of transcription, resulting in 5-FOA resistance.