Accelerated Publications Cyclization of Globular DNA. Implications for DNA-DNA Interactions in Vivo ² Dorothe ´e Jary ‡,§ and Jean-Louis Sikorav* ,§ Laboratoire Le ´ on Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif-sur-YVette Cedex, France, and SerVice de Biochimie et de Ge ´ ne ´ tique Mole ´ culaire, CEA/Saclay, 91191 Gif-sur-YVette Cedex, France ReceiVed NoVember 20, 1998 ABSTRACT: The rate of cyclization of λ DNA varies over more than 6 orders of magnitude, from 3.2 × 10 -7 s -1 to 2 s -1 , in a Tris-EDTA buffer as a function of spermidine concentration. This variation is strictly correlated with the conformation of the chain. The highest rates are obtained when the chain is collapsed into a dense globular state. The effective concentration of the chain ends in the reaction is then 87 000-fold greater than in the random coil state. These results show that DNA globularity must be taken into account to understand biological processes involving intramolecular DNA-DNA interactions. Long DNA molecules are in globular states in cells and viruses (1). These globular states are characterized by an overall DNA conformation which is much denser than in the random coil state. A central issue in molecular biology is to understand the consequences of the globularity of DNA on such fundamental processes as chromosome condensation, DNA replication and recombination, and gene expression. This is a complex problem, as witnessed by the large number of components participating in the compaction process, and by the diversity of globular states found in vivo (in eukaryotic cells, for instance, the density of the globular states is regulated in a subtle manner during the cell cycle: it is minimal during interphase and maximal during mitosis). Many years ago, Lerman discovered that in very dilute solutions, phage DNA molecules can collapse from a random coil state to a compact globular state (2). The globular states of DNA obtained in vitro are of great biological interest since they can serve as simple models to investigate the properties of the more complex globular states of DNA that prevail in vivo. Physically, DNA collapse is due to the presence of net-attractive intramolecular interactions between the seg- ments of the molecule. Agents promoting DNA collapse are called condensing agents, and include multivalent cations such as the polyamines spermidine (3+) or spermine (4+) (reviewed in 3). The attractive forces induced by condensing agents can also act at the intermolecular level, giving rise to a multimolecular aggregation of long or short DNA chains. DNA condensation is a generic term that refers to the formation of a DNA-rich state of high density, and therefore applies both to aggregation of many molecules and to the compaction of a single molecule (4). There exist numerous biological processes involving DNA-DNA interactions. Previous studies have shown that DNA condensation can greatly affect such processes. The rate of DNA renaturation, for instance, and of DNA strand- exchange without protein is greatly accelerated by DNA condensation (5, 6). DNA condensation also favors an extensive catenation of circular DNA chains by DNA topoisomerases (7). In these examples, the condensing agent operates by a multimolecular aggregation of the DNA molecules. We show here by studying the cyclization of the DNA of bacteriophage λ that intramolecular DNA-DNA ² This work has been supported by a grant from the Ministe `re de l’Enseignement Supe ´rieur et de la Recherche (ACC-SV N°5). * To whom correspondence should be addressed. Telephone: 01 69 08 66 43. Fax: 01 69 08 90 52. E-mail: sikorav@jonas.saclay.cea.fr. ‡ Laboratoire Le ´on Brillouin. § Service de Biochimie et de Ge ´ne ´tique Mole ´culaire. © Copyright 1999 by the American Chemical Society Volume 38, Number 11 March 16, 1999 10.1021/bi982770h CCC: $18.00 © 1999 American Chemical Society Published on Web 02/24/1999