REGULATION OF STRINGENT MITOCHONDRIAL TRANSCRIPTION IN YEAST FOLLOWING AMINO-ACID DEPRIVATION

In the yeast Saccharomyces cerevisiae, a phenotypically identical stri ngent response is induced by either nutritional downshift or starvatio n for a required auxotrophic amino acid (aa); in each case, the respon se selectively includes transcriptional curtailment for the mitochondr ial (mt) genome. We have shown previously that the downshift-induced m t stringent response is governed by changing cellular cyclic AMP (cAMP ) levels, via a mt cAMP-dependent protein kinase. In contrast, we demo nstrate here that cAMP levels are not altered in yeast following starv ation for a required aa, and we use in vitro mt transcription assays w ith organelles from wild-type and mutant strains to confirm that the a a starvation-induced mt stringent response is not governed by cAMP. Ra ther, 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 syst ems.