DNA looping.

DNA-looping mechanisms are part of networks that regulate all aspects of DNA metabolism, including transcription, replication, and recombination. DNA looping is involved in regulation of transcriptional initiation in prokaryotic operons, including ara, gal, lac, and deo, and in phage systems. Similarly, in eukaryotic organisms, the effects of enhancers appear to be mediated at least in part by loop formation, and examples of DNA looping by hormone receptor proteins and developmental regulatory proteins have been found. In addition, instances of looped structures have been found in replication and in recombination in both prokaryotes and eukaryotes. DNA loop formation may have different functions in different cellular contexts; in some cases, the loop itself is requisite for regulation, while in others the increase in the effective local concentration of protein may account for the effects observed. The ability of DNA to form loops is affected by the distance between binding sites; by the DNA sequence, which determines deformability and bendability; and by the presence of other proteins that exert an influence on the conformation of a particular sequence. Alteration of the stability of DNA loops and/or protein-DNA binding by extra- or intracellular signals provides responsivity to changing metabolic or environmental conditions. The fundamental property of site-specific protein binding to DNA can be combined with protein-protein and protein-ligand interaction to generate a broad range of physiological states.