A multisubunit transcription factor essential for accurate initiation by RNA polymerase II.

We have fractionated rat liver and identified a set of transcription factors that are essential for accurate initiation of transcription by RNA polymerase II. These factors were resolved into three distinct enzyme fractions. The factors in two of these fractions have been purified to apparent homogeneity. One fraction contains a single, 35-kDa transcription factor, designated alpha (Conaway, J. W., Bond, M. W., and Conaway, R. C. (1987) J. Biol. Chem. 262, 8293-8297). Here we show that the other fraction contains a transcription factor, designated beta gamma, with a native molecular mass of approximately 250 kDa. This factor is composed of 67 kDa (beta) and 31 kDa (gamma) polypeptides. Both the beta and gamma polypeptides are required for runoff transcription. Comparison of purified transcription factors alpha and beta gamma reveals that (i) both factors act at initiation and (ii) neither alpha nor beta gamma is required for assembly of an initial complex that commits the template to be transcribed by RNA polymerase II.