Analysis of transcriptional initiation of yeast mitochondrial DNA in a homologous in vitro transcription system.

We have developed an in vitro transcription system for yeast mitochondrial rRNA genes. Using highly purified yeast mitochondrial RNA polymerase and bacterial plasmids carrying DNA segments containing the mitochondrial rRNA sites of transcriptional initiation, we have been able to demonstrate correct initiation of transcription in vitro. By directly sequencing the transcription products, we show that transcription in vitro of both the 14S and 21S rRNAs is initiated at precisely the same site as it is in vivo. Transcription of the rRNA genes is highly sensitive to ionic strength and RNA polymerase concentration. Additional factors or modified conditions may be necessary to permit accurate transcription of mitochondrial protein genes.