%0 Journal Article
%T DNA loops and semicatenated DNA junctions
%A Claire Gaillard
%A Fran£¿ois Strauss
%J BMC Biochemistry
%D 2000
%I BioMed Central
%R 10.1186/1471-2091-1-1
%X Formation of Form X was found to occur upon reassociation of the strands of a DNA fragment containing a tract of poly(CA) ¡¤ poly(TG), in a process strongly stimulated by the nuclear proteins HMG1 and HMG2. By inserting Form X into DNA minicircles, we show that the DNA strands do not run fully side by side but instead form a DNA knot. When present in a closed DNA molecule, Form X becomes resistant to heating to 100¡ãC and to alkaline pH.Our data strongly support a model of Form X consisting in a DNA loop at the base of which the two DNA duplexes cross, with one of the strands of one duplex passing between the strands of the other duplex, and reciprocally, to form a semicatenated DNA junction also called a DNA hemicatenane.Alternative DNA conformations are of particular interest as potential signals to mark important sites on the genome, contrasting with the rather uniform structure of the classical B-form DNA double helix which shows little variation as a function of its nucleotide sequence. The structural variability of CA microsatellites is particularly pronounced. These are repetitive poly (CA) ¡¤ poly (TG) DNA sequences spread in all eukaryotic genomes as tracts of up to 60 base pairs long, sometimes longer, which are particularly abundant in the human genome where they are present in about 105 copies [1]. Because of this abundance, combined with their frequent length variability between different individuals, they have served as a very useful source of markers in human genetics. Many in vitro studies have shown that the structure of poly (CA) ¡¤ poly (TG) can vary markedly from the classical right-handed DNA double helix and adopt diverse conformations (for a review see e.g. [2] and references therein), the best known of which being left-handed Z-DNA [3,4]. In the course of our work with DNA fragments containing this repetitive sequence we have observed the formation of several alternative structures which appeared as retarded bands upon gel electrophoresis. While
%U http://www.biomedcentral.com/1471-2091/1/1