- The C-terminal repeat domain of RNA polymerase II largest subunit is essential in vivo but is not required for accurate transcription initiation in vitro. Zehring, W.A., Lee, J.M., Weeks, J.R., Jokerst, R.S., Greenleaf, A.L. Proc. Natl. Acad. Sci. U.S.A. (1988)