Stereoalignment requirements for activation of transcription by the simian virus 40 enhancer

We investigated the contribution to the enhancer activity of protein-protein interactions between specific trans-acting factors bound to neighbouring sequence elements of the SV40 early enhancer-promoter region. To this end, we altered the distance between the SV40 enhancer-promoter elements by inserting systematically increasing lengths of spacer DNA. We show here that the level of transcription from the SV40 early promoter decreases with the spacing between enhancer and promoter regions. In the case of insertions shorter than 125 bp, the promoter activity exhibits a strong dependence of insertion lengths of multiples of about 10 bp. This periodic effect is no longer observed for layer insertions, reflecting the torsional flexibility of DNA. These data provide evidence that, between promoter and/or enhancer elements, periodic interactions of transcriptional nucleoprotein complexes exist and stereospecific alignments are necessary to obtain an efficient initiation of transcription from the SV40 early promoter.