Table I.

DNA Looping in Prokaryotic and Eukaryotic Transcriptional Regulation

Molecule or locus	Mode of action	Wild type loop lengths (bp)
Lac repressor83	Repression	92, 401
AraC85	Repression and activation	210
Gal repressor85	Repression	115
Deo repressor85	Repression	270, 599, 869
Nag repressor86	Repression	93
NtrC87	Activation	110–140
λ repressor84,88	Repression and activation	~2400
XylR89	Activation	~150
PapI87,90	Activation	~100
β-globin locus91,92	Activation	40,000–60,000
RXR93	Activation	~0–500
SpGCF194	Activation, domain intercommunication	~0–2500
HSTF95	Activation	23
p5396	Repression and activation	50–3000
Sp197–99	Activation	~1,800
c-Myb and C/EBP100	Activation	~80

Loop lengths and mechanisms of action of some of the best known looping systems in bacteria and eukaryotes. Note that these loop lengths suggest tightly bent configurations since the in vitro measured persistence length is 150 bp.