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 α (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 βγ, with a native molecular mass of approximately 250 kDa. This factor is composed of 67 kDa (β) and 32 kDa (γ) polypeptides. Both the β and γ polypeptides are required for runoff transcription. Comparison of purified transcription factors α and βγ reveals that (i) both factors act at initiation and (ii) neither α nor βγ is required for assembly of an initial complex that commits the template to be transcribed by RNA polymerase II.