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.