Mitochondria share a version of their RNA polymerase with the nucleus, as shown by Julia Kravchenko, Igor Rogozin, Eugene Koonin (NIH, Bethesda, MD), and Peter Chumakov (Cleveland Clinic Foundation, Cleveland, OH). This new fourth mammalian nuclear RNA polymerase adds a new dimension to mRNA transcription.
Until now, mRNA transcription was considered the sole domain of RNA polymerase II (RNAP-II). While trying to determine why some transcripts were still abundant when RNAP-II was inhibited, Kravchenko et al. found that an alternatively spliced version of the mitochondrial RNAP (mtRNAP) was responsible. Although nuclear DNA–encoded, mtRNAP is used solely by mitochondria. But the new spliced version (spRNAP-IV) was instead imported into the nucleus and was shown to transcribe several nuclear mRNAs whose genes share a short putative promoter sequence not recognized by RNAP-II.
The authors estimate that the number of genes transcribed by spRNAP-IV is on the order of a thousand (although some might be co-regulated by RNAP-II). At least some of these genes encode vital proteins, as loss of spRNAP-IV slows proliferation and leads to cell death. Koonin hypothesizes that spRNAP-IV “somehow allows the cell to simultaneously regulate mitochondrial function and other aspects of cell behavior,” possibly including apoptosis.