Simian virus 40 enhancer increases number of RNA polymerase II molecules on linked DNA

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Abstract
Since the discovery of transcription enhancer sequences in the genomes of the DNA viruses simian virus 40 (SV40)1–3 and polyoma virus4, these elements have been shown to play an important part in the control of both viral and cellular gene expression5–10. Enhancer elements act in cis to increase the amount of RNA produced from linked genes in a manner largely independent of distance and orientation11–3,11,12. The mechanisms by which enhancers act are not understood; in particular, it is not known whether the enhancer-dependent increase in the level of stable RNA reflects an increase in the rate of transcription. To address this question, we have used an in vitro nuclear transcription assay13,14 to examine the effect of the SV40 enhancer on transcription of cloned human β-globin genes transiently introduced into HeLa cells15. We show here that the SV40 enhancer acts at least in part to increase the number of RNA polymerase II molecules transcribing the linked gene.