RNA Polymerase II Transcription Factor SIII. I. Identification, Purification, and Properties

A transcription factor that stimulates synthesis of accurately initiated transcripts by RNA polymerase II has been identified and purified to apparent homogeneity from rat liver nuclear extracts. This factor, which we designate SIII, has a native molecular mass of approximately 140 kDa and is composed of three polypeptides of 110, 18, and 15 kDa. All three polypeptides are required to reconstitute SIII transcriptional activity. SIII appears distinct from previously identified mammalian transcription factors.