The overall objective of this work is to examine the structural components of a complex mammalian enzyme, RNA polymerase II. Precursor-product relationships between subunits will be examined by peptide fingerprinting and immunological cross reactivity; these studies will involve preparative isolation of subunits. Subunit depleted derivatives of polymerase II will be measured. Polymerase II will be modified with group specific reagents and substrate analogues to localize residues critical to catalysis. The kinetics of reaction and the effect of catalysis will be examined. The subunit topography of the enzyme and modified derivatives will be probed using cross-linking reagents.