Reconstitution of Yeast and Arabidopsis RNA Polymerase α-like Subunit Heterodimers*

Two subunits of about 36–44 kDa and 13–19 kDa in the eukaryotic nuclear RNA polymerases share limited amino acid sequence similarity to the α subunit in Escherichia coliRNA polymerase. The α subunit in the prokaryotic enzyme has a stoichiometry of 2, but the stoichiometry of the α-like subunits in the eukaryotic enzymes is not entirely clear. To gain insight into the subunit stoichiometry and assembly pathway for eukaryotic RNA polymerases, in vitro reconstitution experiments have been carried out with recombinant α-like subunits from yeast and plant RNA polymerase II. The large and small α-like subunits from each species formed stable heterodimers in vitro, but neither the large or small α-like subunits formed stable homodimers. Furthermore, mixed heterodimers were formed between corresponding subunits of yeast and plants, but were not formed between corresponding subunits in different RNA polymerases from the same species. Our results suggest that RNA polymerase II α-like heterodimers may be the equivalent of α homodimers found inE. coli RNA polymerase.