Regulation of stringent mitochondrial transcription in yeast following amino-acid deprivation

In the yeast Saccharomyces cerevisiae, a phenotypically identical stringent response is induced by either nutritional downshift or starvation for a required auxotrophic amino acid (aa); in each case, the response selectively includes transcriptional curtailment for the mitochondrial (mt) genome. We have shown previously that the downshift-induced mt stringent response is governed by changing cellular cyclic AMP (cAMP) levels, via a mt cAMP-dependent protein kinase. In contrast, we demonstrate here that cAMP levels are not altered in yeast following starvation for a required aa, and we use in vitro mt transcription assays with organelles from wild-type and mutant strains to confirm that the aa starvation-induced mt stringent response is not governed by cAMP. Rather, such stringent organellar transcriptional attenuation may result from altered availability of an unidentified small molecule which is probably a product of the cytoplasmic and/or mt protein synthesis systems.