Nucleic Acids Research, 2000, Vol. 28, No. 10 2187-2193
© 2000 Oxford University Press
Hemicatenanes form upon inhibition of DNA replication
Laboratoire de Génétique Moléculaire, Ecole Normale Supérieure, 46 rue dUlm, 75230 Paris Cedex 05, France
Plasmid DNA incubated in interphase Xenopus egg extracts is normally assembled into chromatin and then into synthetic nuclei which undergo one round of regulated replication. During a study of restriction endonuclease cut plasmid replication intermediates (RIs) by the BrewerFangman 2D gel electrophoresis technique, we have observed the formation of a strong spike of X-shaped DNA molecules in extracts that otherwise yield very little or no RIs. Formation of these joint molecules is also efficiently induced in replication-competent extracts upon inhibition of replication fork progression by aphidicolin. Although their electrophoretic properties are quite similar to those of Holliday junctions, 2D gels of doubly cut plasmids show that these junctions can link two plasmid molecules at any pair of DNA sequences, with no regard for sequence homology at the branch points. Neutralneutralalkaline 3D gels show that the junctions only contain single strands of parental size and no recombinant strands. A hemicatenane, in which one strand of a duplex is wound around one strand of another duplex, is the most likely structure to account for these observations. The mechanism of formation of these novel joint DNA molecules and their biological implications are discussed.
* To whom correspondence should be addressed. Tel: +33 1 44 32 39 20; Fax: +33 1 44 32 39 41; Email: hyrien@wotan.ens.fr
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