Subunit Structure Differences in RNA Polymerase II Purified from Ungerminated versus Germinated Wheat Embryos

DNA-dependent RNA polymerase II (RNAP II) was purified from wheat embryos germinated for 0, 12, 24, and 36 hours and examined with several polyacrylamide gel electrophoretic systems. A changing electrophoretic pattern of RNAP II was observed on nondenaturing polyacrylamide gels. Subunit structure analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated that from ungerminated embryos, RNAP IIA was almost exclusively obtained which has a subunit structure identical to that established for wheat germ RNAP II previously (Jendrisak, Burgess 1977 Biochemistry 16: 1959-1964). Twelve polypeptides with molecular weights x 10(-3) of 220, 140, 42, 40, 27, 25, 21, 20, 17.8, 17.0, 16.3, and 16.0 were routinely found to be associated with the purified enzyme. From embryos germinated for 36 hours, RNAP IIB was almost exclusively obtained which has a largest subunit of 180,000 mol wt instead of 220,000. From embryos germinated for 24 hours, an approximately equimolar mixture of RNAP IIA and IIB was obtained. Peptide maps of the 220,000 and 180,000 mol wt polypeptides of RNAP IIA and IIB were virtually identical, indicative of a precursor-product relationship for the two polypeptides. In addition to these results, SDS-PAGE indicated that the stoichiometry of the 27,000 mol wt polypeptide increased at the expense of the 25,000 mol wt polypeptide during germination and concomitantly with the appearance of the 180,000 molecular weight polypeptide. No modifications (e.g. gain, loss, or altered mobilities on analytical gels) in any of the other RNAP II subunits were observed in enzyme purified from embryos after various times of germination as determined by a variety of electrophoretic analyses under denaturing conditions.