HAP1 IS NUCLEAR BUT IS BOUND TO A CELLULAR FACTOR IN THE ABSENCE OF HEME

The activity of the yeast transcriptional activator HAP1 is controlled by heme. Previously, it has been shown that a heme-responsive domain containing multiple repeats of a conserved motif blocks DNA binding in the absence of heme. In this report, we show that HAP1 is sequestered in a high molecular weight complex in the absence of heme. Titration of the high molecular weight complex by addition of a non-DNA-binding form of HAP1 allows the protein to form dimeric complexes in the absen ce of heme in vitro and acquires partial transcriptional activity in v ivo. The results indicate that one or more cellular factor(s) complexe s with HAP1 and represses its activity in the absence of heme. Deletio n of the heme domain prevents sequestration of HAP1 in the high molecu lar weight complex. We discuss these findings in a model that postulat es that the heme domain of HAP1 can interact with other cellular facto rs to regulate HAP1.