Atomic force microscopic demonstration of DNA looping by GalR and HU.

Regulation of gene transcription in both prokaryotes and eukaryotes involves formation of various DNA-multiprotein complexes of higher order structure through communication between distant regions of DNA. The communication between distant DNA sites occurs by interaction between proteins bound to the sites by looping out the intervening DNA segments. The repression of transcription of two overlapping promoters of the gal operon in Escherichia coli requires Gal repressor (GalR) and the histone-like protein HU. Both in vivo and in vitro data support a proposed HU containing complex responsive to induction in which GalR molecules bound to two distant operator sites interact by looping out DNA. We successfully applied atomic force microscope (AFM) imaging to visualize galDNA complexes with proteins. We report GalR mediated DNA looping in which HU plays an obligatory role by helping GalR tetramerization. Supercoiling of DNA, which is also critical for GalR action, may stabilize the DNA loops by providing an energetically favorable geometry of the DNA.