Nuclear two-hybrid systems for autoactivating bait proteins. (A) RNA Pol III system (511). Through its fusion with the Gal4 DBD, an autoactivating bait (AuX) protein is tethered to the Gal1 upstream activating sequence, located downstream of the reporter gene SNR6. Interaction of the AuX bait with a prey protein (Y) fused to the τ138 subunit of transcription factor III C (TFIIIC) brings the RNA polymerase III holoenzyme to SNR6. Transcription of SNR6 leads to survival under temperature-restrictive (inactive snR6-A62G) conditions. (B) Repressed transactivator system (259). In the repressed transactivator (RTA) system, association of an autoactivating AuX bait with the prey protein Y, attached to the repressor Tup1, inhibits transcription of the reporter gene URA3. This results in survival of the cell on 5-FOA, a substrate for the production of the toxic compound 5-fluorouracil by the gene product of URA3. Pol, polymerase; TF, transcription factor; Y, prey; AuX, autoactivating bait; AD, activation domain; DBD, DNA-binding domain; RD, repressor domain; UAS, upstream activating sequence.