The nonphosphorylated form of RNA polymerase II preferentially associates with the preinitiation complex.

The two forms of RNA polymerase II that exist in vivo, phosphorylated (IIO) and nonphosphorylated (IIA), were purified to apparent homogeneity from HeLa cells. The nonphosphorylated form preferentially binds to the preinitiation complex. RNA polymerase II in the complex was converted by a cellular protein kinase to the phosphorylated form.