DNA looping and Sp1 multimer links: a mechanism for transcriptional synergism and enhancement.
Using conventional and scanning transmission electron microscopy, we have examined the physical basis of long-range enhancer effects between distal and proximal elements in a eukaryotic promoter. Specifically, we have studied binding of human transcription factor Sp1 to 10-base-pair G+C-rich elements ("GC boxes") located at -100 and +1700 relative to the RNA start site. It was previously observed that the distantly located site functions in synergism with the promoter-proximal site to strongly activate transcription in vivo. Here we demonstrate that this synergism is likely to be a direct consequence of interactions between remote and local Sp1, the remote Sp1 translocated to the promoter by a DNA loop. Scanning transmission electron microscopy shows that Sp1 initially forms a tetramer and subsequently assembles multiple tetramers stacked in register at the DNA loop juncture. This unexpected finding not only provides the physical basis for loop formation but also defines a biological process leading to strongly increased concentration of activator protein at the promoter. The mechanism may unify the problem of transcriptional activation by removing enhancer action as a separate class of regulatory activity.
Mastrangelo
I A
IA
Biology Department, Brookhaven National Laboratory, Upton, NY 11973.
Courey
A J
AJ
Wall
J S
JS
Jackson
S P
SP
Hough
P V
PV
D004247:DNA / Q000378:metabolism* / Q000648:ultrastructure; D004742:Enhancer Elements, Genetic; D005786:Gene Expression Regulation; D066298:In Vitro Techniques; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008855:Microscopy, Electron, Scanning; D010957:Plasmids; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D012045:Regulatory Sequences, Nucleic Acid; D016329:Sp1 Transcription Factor / Q000502:physiology*; D014158:Transcription, Genetic
D046911:Macromolecular Substances; D016329:Sp1 Transcription Factor; D004247:DNA