The characterization of yeast mitochondrial RNA polymerase. A monomer of 150,000 daltons with a transcription factor of 70,000 daltons.

Transcription in organelles is regulated by both organellar and nuclear mechanisms. In order to study further the control of organellar transcription, we have purified and characterized the RNA polymerase from mitochondria of Saccharomyces cerevisiae and identified a transcription factor required for promoter recognition. The RNA polymerase can be separated into two forms, selective and nonselective. The nonselective form was purified over 11,000-fold and appears to be active as a monomer with a molecular weight of 150,000. The Mr 150,000 polypeptide acts as a core RNA polymerase, and an Mr 70,000 polypeptide appears to confer selectivity for the promoter upon this core. The Mr 70,000 transcription factor binds specifically to the mitochondrial initiation site in the absence of polymerase and decreases nonselective initiation by the polymerase. The Mr 150,000 polymerase is immunologically related to an Mr 145,000 protein purified from yeast as a primase, although it is thought to be a functional unit of mitochondrial RNA polymerase (Kelly, J. L., and Lehman, I. R. (1986) J. Biol. Chem. 261, 10340-10347). Antibodies to the Mr 145,000 protein inhibit transcription by the mitochondrial RNA polymerase purified here.