A team headed by Craig Pikaard, Ph, D., Washington University professor of biology in Arts & Sciences, has discovered a fourth kind of RNA polymerase found only in plants and speculated to have been a plant feature for more than 200 million years.
RNA polymerase is an enzyme, or protein machine, essential for carrying out functions of cells and for expression of biological traits. It does its job by copying a template of DNA genetic information in order to make RNAs that encode proteins or that function directly in the cell.
Biologists have studied three kinds of RNA polymerase for decades in organisms ranging from brewer's yeast to humans. In all eukaryotes, the RNA polymerases Pol I, II, and III perform the same distinct , though separate, functions in different species.
But then along came Pol IV. Pikaard first noticed the evidence for a fourth polymerase when analyzing gene sequences after Arabidopsis thaliana , the "laboratory rat" of the plant world, was sequenced in 2001. It originally looked to him like an alternative form of either Polymerase I (Pol I), which makes the largest of the ribosomal RNAs, Pol II which makes RNAs for protein-coding genes, or Pol III, a specialist in making the shortest of the ribosomal RNAs and tRNAs.
The big 'subunit'
He and his colleagues looked specifically at two polypeptides that would be the key subunits if the fourth polymerase were functional, namely the largest and second largest subunits, what Pikaard refers to as the catalytic, or "business end" of any known polymerase.