Transcription of the yeast mitochondrial genome requires cyclic AMP

Using various mutant strains and nutritional manipulations, we investigated a potential role for cyclic AMP (cAMP) in the regulation of mitochondrial (mt) gene expression in the yeast Saccharomyces cerevisiae. In RAS mutants known to have either abnormally low or high cellular levels of this nucleotide, we show that both mt transcription rate and overall mt transcript levels vary directly with cellular cAMP levels. We further show that nutritional downshift of actively growing cells causes a severe, rapid fall in cAMP levels, and that this fall is concomitant with the stringent mt transcriptional curtailment that we and others have previously shown to follow this nutritional manipulation. In in vitro mt transcription assays using intact organelles from downshifted and actively growing cells, stringently curtailed mt gene expression can be restored to 75% of control levels by addition of cAMP to the assay mix. Consistent with these observations a RAS2vall9 mutant strain, which cannot adjust cAMP levels in response to external stimuli, shows no mt stringent response following nutritional downshift. We also demonstrate a significant but transient increase in both mt transcript levels and mt transcription rate following shift of actively respiring wild-type cells to glucose-based medium, a manipulation known to cause a short-lived pulse of cAMP in yeast; similar manipulation of the RAS2vall9 mutant strain generates no such response. Taken together all these observations indicate that cellular cAMP levels are involved in 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 that cAMP may influence mt transcription via a mt cAMP-dependent protein kinase. These results link mt gene expression with mechanisms governing growth control and nutrient adaptation in yeast, and they provide a means by which mt gene expression might be coordinated with that of related nuclear genes.