Transcription of muscle actin genes by a nuclear form of mitochondrial RNA polymerase.

Actins are the major constituent of the cytoskeleton. In this report we present several lines of evidence that muscle actin genes are transcribed by nuclear isoform of mitochondrial RNA polymerase (spRNAP-IV) whereas the non-muscle actin genes are transcribed by the conventional RNA polymerase II (PolII). We show that mRNA level of muscle actin genes are resistant to PolII inhibitors α-amanitin and triptolide as well as insensitive to knockdown of PolII but not to knockdown of spRNAP-IV, in contrast to non-muscle actin genes in several cell lines. Similar results are obtained from nuclear run-on experiments. Reporter assay using muscle actin or PolII gene promoters also demonstrate the differential sensitivity to PolII inhibitors. Finally, chromatin-immunoprecipitation experiment was used to demonstrate that spRNAP-IV is associated with promoter of muscle actin genes but not with that of non-muscle gene and knockdown of spRNAP-IV depleted this polymerase from muscle actin genes. In summary, these experiments indicate that the two types of actin genes are transcribed by different transcription machinery. We also found that POLRMT gene is transcribed by spRNAP-IV, and actin genes are sensitive to oligomycin, suggesting a transcription coupling between mitochondria and nucleus.