TRANSCRIPTION OF THE YEAST MITOCHONDRIAL GENOME REQUIRES CYCLIC-AMP

Using various mutant strains and nutritional manipulations, we investi gated a potential role for cyclic AMP (cAMP) in the regulation of mito chondrial (mt) gene expression in the yeast Saccharomyces cerevisiae. In RAS mutants known to have either abnormally low or high cellular le vels of this nucleotide, we show that both mt transcription rate and o verall mt transcript levels vary directly with cellular cAMP levels. W e further show that nutritional downshift of actively growing cells ca uses a severe, rapid fall in cAMP levels, and that this fall is concom itant with the stringent mt transcriptional curtailment that we and ot hers have previously shown to follow this nutritional manipulation. In in vitro mt transcription assays using intact organelles from downshi fted and actively growing cells, stringently curtailed mt gene express ion can be restored to 75% of control levels by addition of cAMP to th e assay mix. Consistent with these observations a RAS2vall9 mutant str ain, which cannot adjust cAMP levels in response to external stimuli, shows no mt stringent response following nutritional downshift. We als o demonstrate a significant but transient increase in both mt transcri pt levels and mt transcription rate following shift of actively respir ing wild-type cells to glucose-based medium, a manipulation known to c ause a short-lived pulse of cAMP in yeast; similar manipulation of the RAS2vall9 mutant strain generates no such response. Taken together al l these observations indicate that cellular cAMP levels are involved i n the regulation of mt transcription in yeast. Moreover, the lack of a mt stringent transcriptional response following downshift in a strain in which the BCY1 gene had been insertionally inactivated suggests th at cAMP may influence mt transcription via a mt cAMP-dependent protein kinase. These results link mt gene expression with mechanisms governi ng growth control and nutrient adaptation in yeast, and they provide a means by which mt gene expression might be coordinated with that of r elated nuclear genes.