Stimulation of in vitro transcription from the SV40 early promoter by the enhancer involves a specific trans‐acting factor

A nuclear extract prepared from HeLa cells has been used to study in vitro the transcription of the SV40 early promoter. The deletion of the enhancer results in a strong decrease of transcription, with spermidine and MgCl2 being critical variables in the transcription reactions. Furthermore a competition assay indicates that the stimulation by the enhancer is due to a specific trans‐acting factor(s) which acts on it. This factor appears not to interact with SV40 or adenovirus‐2 major late upstream (distal) promoter sequences, and its ability to bind to the enhancer is diminished by mutations known to decrease enhancer function in vivo and in vitro.