Transcription factor SIII: a novel component of the RNA polymerase II elongation complex.

A growing body of evidence suggests that the elongation stage of eukaryotic messenger RNA synthesis is a major site for the regulation of gene expression. In the process of investigating the mechanism of promoter-specific transcription by RNA polymerase II, we recently identified and purified a novel transcription factor that controls RNA chain elongation. This factor, which we have designated SIII, has a native molecular mass of 140 kDa and is composed of three polypeptides of 110, 18, and 15 kDa. Results of renaturation experiments indicate that all three polypeptides are required to reconstitute SIII transcriptional activity. A variety of evidence from mechanistic studies reveals that SIII dramatically stimulates the rate of RNA chain elongation by RNA polymerase II, most likely through a direct interaction with transcribing polymerase. Here we describe the properties of SIII as well as its role in regulating the activity of the RNA polymerase II elongation complex.