Transactivation by Rtg1p, a basic helix-loop-helix protein that functions in communication between mitochondria and the nucleus in yeast

Rtg1p is a basic helix-loop-helix transcription factor in the yeast Saccharomyces cerevisiae that is required for basal and regulated expression of CIT2, the gene encoding a peroxisomal isoform of citrate synthase. In respiratory incompetent ρ° petite cells, CIT2 transcription is elevated as much as 30-fold compared with respiratory competent ρ⁺ cells. Here we provide evidence that Rtg1p interacts directly with a CIT2 upstream activation site (UAS(r)) and that the ρ°/ρ⁺ regulation is not due to a change in the levels of Rtg1p. A fusion protein consisting of the DNA binding domain of Gal4p fused to the NH₂ terminus of the full-length wild-type Rtg1p was able to transactivate an integrated Latz reporter under control of the Gal4p-responsive GAL1 UAS(G) in a ρ°/ρ⁺dependent manner. Other Gal4p fusions to deletions or mutations of Rtg1p indicate that the helix-loop- helix domain is essential for transactivation. Regulated expression of CIT2 also requires the RTG2 gene product. The Gal4-Rtg1p fusion was unable to transactivate the Latz reporter gene in a strain deleted for RTG2, suggesting that the RTG2 product does not act independently of Rtg1p in the ρ°/ρ⁺ transcriptional response.