Analysis of transcription asymmetries along the tRNA^E-COB operon: Evidence for transcription attenuation and rapid RNA degradation between coding sequences

Mitochondrial gene expression in yeast is believed to be regulated predominantly at the post-transcriptional level. However, the contribution of mitochondrial transcription and RNA-turnover rates to differential gene regulation is still largely unknown. Mitochondrial run-on transcription and hybrid selection assays showed that some of the multigenic transcription units of the mitochondrial genome are transcribed evenly, whereas others are transcribed asymmetrically, with higher transcription rates for promoter-proximal genes, than for promoter-distal genes. The tRNA^E-cytochrome b (COB) operon was analyzed in detail to investigate the mechanisms underlying transcription rate asymmetries in yeast mitochondria. We showed that a drop in transcription rates occurs in a particular region between the coding sequences and is independent of the coding sequence of the downstream COB gene. Deletion of the region between tRNA^E and COB coding sequences decreases the drop in transcription rates. Deletion of the nuclear gene encoding the Pet 127 protein, which is involved in mitochondrial RNA 5′ processing and degradation, also partially relieves transcriptional asymmetry. Therefore, asymmetry is probably due to a combination of attenuated transcription at specific sites between the coding sequences and very rapid RNA degradation.