Altered gene expression correlates with DNA structure.
We examined the participation of triplex DNA structure in gene regulation using a poly(dG)-poly(dC) sequence as a model. We show that a poly(dG)-poly(dC) sequence, which can adopt an intramolecular dG.dG.dC triplex under superhelical strain, strongly augments gene expression when placed 5' to a promoter. The activity of this sequence exhibits a striking length dependency: dG tracts of 27-30 bp augment the expression of a reporter gene to a level comparable to that observed with the polyoma enhancer in mouse LTK- cells, whereas tracts of 35 bp and longer have virtually no effect. A supercoiled plasmid containing a dG tract of 30 bp competes in vivo for a trans-acting factor as revealed by reduction in the reporter gene transcription driven by the (dG)29/promoter of the test plasmid, while dGs of 35 bp and longer in the competition plasmid failed to compete. In purified supercoiled plasmid DNA at a superhelical density of -0.05, dG tracts of 32 bp and longer form a triplex, whereas those of 30 bp and shorter remain double-stranded under a PBS solution. These results suggest that a localized superhelical strain can exist, at least transiently, in mouse LTK- cells, and before being relaxed by topoisomerases this rapidly induces dG tracts of 35 bp and longer to adopt a triplex preventing the factor from binding. Thus, these data suggest that a poly(dG)-poly(dC) sequence can function as a negative regulator by adopting an intramolecular triple helix structure in vivo.
Kohwi
Y
Y
La Jolla Cancer Research Foundation, California 92037.
Kohwi-Shigematsu
T
T
D000818:Animals; D001483:Base Sequence; D001667:Binding, Competitive; D004247:DNA / Q000737:chemistry*; D005786:Gene Expression Regulation; D006801:Humans; D007739:L Cells; D051379:Mice; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D011401:Promoter Regions, Genetic; D015534:Trans-Activators / Q000235:genetics
D015534:Trans-Activators; D004247:DNA