Purification of a new protein stimulating RNA polymerase II from ehrlich ascites tumor cells: Comparison with proteins purified before

A protein factor named S-I(b) that stimulates the activity of RNA polymerase II was purified from Ehrlich ascites tumor cells. The final preparation gave a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein has a molecular weight of 24,000 and is significantly smaller than S-II and S-II', stimulatory proteins of RNA polymerase II purified previously. However, S-I(b) cross-reacted with antibody against S-II and its peptide map obtained after radio-iodination was identical with that of S-II' (a phosphorylated form of S-II), indicating that S-I(b) and S-II share a common primary structure. S-I(b) is suggested not to be a degradation product of S-II produced during the purification process, and the structural relations between S-II and S-II' are discussed.