A new view of DNA packing in chromosomes was described by Ulrich Laemmli of the University of Geneva in Switzerland. Previously scientists had observed many loops of the DNA extending from a protein scaffold, but they had not detected fixed binding sites between the DNA and the scaffold. Now Laemmli reports specific, strategically located attachment sites. For example, fruit flies have a cluster of histone genes that is repeated about a hundred times. Each copy of the cluster forms one loop. Laemmli finds the same pattern for other genes–but the loops are of different lengths.
In each case, the attachment sites fall near the DNA segments, called promoters, that initiate gene activity. Thus the loop may represent a single gene or a set of adjacent genes that are coordinately expressed. Laemmli has recently determined that the major protein in the scaffold is an enzyme called topoisomerase II.
This enzyme had previously been recognized for its activity in untangling DNA. Laemmli therefore suggests that the attachment sites he has identified are the sites where topoisomerase II binds DNA. The function of the attachment sites is still uncertain–they may maintain order in chromosome packing, may control the supercoiling of the chromosome or may form compartments for gene expression.