Yeast Cyc8p and Tup1p proteins function as coactivators for transcription of Stp1/2p-dependent amino acid transporter genes

The yeast Cyc8p-Tup1p complex is known to serve primarily as a transcriptional corepressor in a variety of biological processes. However, less is known about its function as a coactivator. Herein, we found tryptophan transporter genes, TAT1 and TAT2, that, when overexpressed, suppressed the slow growth of Δcyc8. We observed that the addition of tryptophan to Δcyc8 cultures partially restored cell growth, and the deletion of CYC8 and TUP1 reduced transcriptional levels of TAT1 and TAT2. Tup1p bound to the promoter region of TAT1 and TAT2 genes that were dependent on STP1 and STP2 (encoding DNA-binding activator proteins) for expression. Similarly, transcription of the other Stp1/2p-dependent amino acid transporter (AAT) genes also required CYC8 and TUP1 gene functions. These data indicate that Cyc8p-Tup1p plays a role as a transcriptional coactivator for AAT genes via Stp1/2p activators and that lowering intracellular tryptophan by CYC8 deletion causes slow growth.