Rpb3, stoichiometry and sequence determinants of the assembly into yeast RNA polymerase II in vivo.

Stoichiometry of the third largest subunit (Rpb3) of the yeast RNA polymerase II is a subject of continuing controversy. In this work we utilized immunoaffinity and nickel-chelate chromatographic techniques to isolate the RNA polymerase II species assembled in vivo in the presence of the His6-tagged and untagged Rpb3. The distribution pattern of tagged and untagged subunits among the RNA polymerase II molecules is consistent with a stoichiometry of 1 Rpb3 polypeptide per molecule of RNA polymerase. Deletion of either alpha-homology region (amino acids 29-55 or 226-267) from the Rpb3 sequence abolished its ability to assemble into RNA polymerase II in vivo.