REGULATION OF MITOCHONDRIAL TRANSCRIPTION DURING THE STRINGENT RESPONSE IN YEAST

In yeast (S. cerevisiae) the stringent response is known to include rapid, selective, and severe transcriptional curtailment for genes specifying cytoplasmic rRNAs and r-proteins. We have shown that transcription of the mitochondrial 21S rRNA gene is also congruently and selectively curtailed during the yeast stringent response. Using an in vitro transcription assay with intact organelles from both rho+ and rho--strains, we show here that the mitochondrial stringent response includes not only transcription of the 21S and 16S rRNA genes, but also that of organellar genes specifying non-mitoribosome-related products. Stringent organellar transcriptional curtailment is identical when cells are starved for a required (marker) amino acid or when they are subjected to nutritional downshift, and the relative level of that transcriptional curtailment following either perturbation is the same in cells growing on fermentative (repressing) or purely respiratory carbon sources. These results confirm that the mechanism governing mitochondrial gene expression during a stringent response is specified outside the organelle, and they show that this transcriptional control mechanism is not immediately subject to glucose repression. In all strains examined, stringent organellar gene expression requires a mitochondrial promoter, suggesting that the regulatory mechanism which functions during the stringent response operates primarily at transcriptional initiation.