Location of DNA and nucleotide binding sites on wheat germ RNA polymerase II

We have investigated the roles of the 13 subunits present in wheat germ RNA polymerase II, using the inhibitors; pyridoxal 5'-phosphate and the periodate oxidation product of adenosine (AOP). Pyridoxal 5'-phosphate is shown to interact with at least part of the DNA binding site as well as the nucleotide binding sites, whereas AOP probably binds to the nucleotide binding sites. Reduction of the enzyme:inhibitor complex with sodium [3H] borohydride and identification of labelled subunits shows that in both cases the inhibitors bind primarily to subunits a and b. We conclude that subunits a and b contain at least part of the catalytic site, but do not rule out possible involvement of other subunits in the various steps of transcription.