The Heme Activator Protein Hap1 Acts as a Transcriptional Repressor
in Saccharomyces cerevisiae.
Thomas Hon, Hee Chul Lee, Li
Zhang
Biochemistry, NYU School of Medicine, 550 1st Avenue, New
York, NY 10016, USA
The heme activator protein Hap1 is known to be a
potent transcriptional activator of many genes encoding functions
required for respiration and for controlling oxidative damage. As an
activator, the activity of Hap1 is stimulated by heme and is highly
sensitive to changes of heme concentration. Our recent data show that
the precise regulation of Hap1 activity by heme is mediated by a
distinctive two-tier mechanism involving multiple Hap1 elements and
multiple molecular chaperones. Remarkably, emerging evidence suggests
that Hap1 also directly represses transcription of many genes, including
HAP1 and MOT3. Evidently, Hap1 represses MOT3 transcription only in
heme-deficient cells while it represses HAP1 transcription in both heme-deficient and heme-sufficient cells. The results suggest that multiple
mechanisms may operate to confer Hap1-mediated transcriptional
repression. Such mechanisms of transcriptional repression are likely to
involve different networks of molecular interactions from those involved
in the two-tier mechanism governing heme regulation of Hap1 as an
activator.
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