In our previous work (Lisowsky et al. 1987; Lisowsky and Michaelis 1988) we have identified two nuclear pet genes of yeast that are required for mitochondrial transcription. In this report we show that one of these pet mutations, pet-ts798, maps in the RP041 gene encoding mitochondrial RNA polymerase. The temperature-sensitive lesion of mutant pet-ts798 can be suppressed by a second nuclear gene RF1023 (mtf1) when inserted into a high copy number plasmid. Our assumption that mtf1 codes for a 40 kDa mitochondrial transciription factor is supported by the fact that the cloned gene acts as an intergenic suppressor of a temperature-sensitive RNA polymerase mutant. A third nuclear gene (mtf2) for mitochondrial transcription was identified by analysing mutant pet-ts3504. The in vitro transcriptional activity of isolated mutant mitochondria is temperature sensitive suggesting the presence of an altered component of transcription inside mitochondria. The defect was confirmed by studies with a transcriptionally active DNA-protein complex and by testing the DNA-binding ability of mitochondrial proteins.