4th Nuclear RNA Polymerase Identified
Topic: Biology
Well remember
central dogma of Biology (DNA => RNA => protein)?
There were 3 known polymerases in mammalian nuclei that copied DNA into RNA (this copying process is known as
transcription).
Pol I: responsible for transcribing
rRNAs (the RNAs that form the catalytic portion of the
ribosome, the machine responsible for translating mRNA into protein - see this entry on
why RNA can support catalytic reactions)
Pol II: responsible for transcribing
mRNA (the common messenger RNA that is translated into proteins) and
snRNA (small nuclear RNAs that are part of the RNA splicing machinery, see entry on
mRNA splicing).
Pol III: responsible for transcribing one rRNA (the 5s rRNA) one snRNA (called U6) and the
tRNAs (tiny t-shaped RNAs that plug into the ribosome and bind both to the mRNA sequence and to the appropriate amino acid - remember amino acids are the lego blocks of proteins).
But wait there is more! Eukaryotic cells are the result of a
symbiotic relation ship between a large primordial cell and mitochondria (see
organelle C). These "mitos" (as some researchers affectionately call them) are thought to be descendants of oxidative alpha-proteobacteria bacteria that could oxidize sugars to water and CO2, as proposed by
Lynn Margulis (click
here for a brief summary of her ideas). Mitochondria have their own DNA that encodes for their own mRNA - and thus they have their own RNA polymerases. The translation of mRNA into protein is slightly different in mitos than in the nucleus, so mitos also have their own tRNA.
OK now you are ready ...
In the latest issue of Nature,
a fourth RNA polymerase has been discovered - named spRNAP-IV, for single-polypeptide RNA Polymerase IV ... or
Pol IV. This protein is actually encoded by a mitochondrial gene! The gene is
alternatively spliced into spRNAP-IV, that is transported to the nucleus and mtRNAP, an RNA polymerase responsible for translating mitochondrial genes. Like Pol II, spRNAP-IV seems to translate genes that encode mRNAs. An interesting side note is that many putative proteins required for mito function are encoded by nuclear genes ... it is thought that these genes once resided in the mitos but then were transfered to the nucleus. Perhaps when the mito genes were "transfered", the nucleus also had to import mito specific RNA polymerases to copy these mito derived genes into mRNA ... and Pol IV is a remnant of that event?
In anycase this study demonstrates that many basic cellular functions are yet to be discovered and it leaves open the question of
what other basic machinery is out there to be found?Ref: Kravchenko et al., Transcription of mammalian messenger RNAs by a nuclear RNA polymerase of mitochondrial origin. Nature 436:735-739
Posted by madscientist39
at 9:03 AM EDT
Updated: Tuesday, 9 August 2005 6:07 PM EDT