Nucleic Acids Research, 2000, Vol. 28, No. 2 472-480
© 2000 Oxford University Press
Nucleoplasmin-mediated chromatin remodelling is required for Xenopus sperm nuclei to become licensed for DNA replication
CRC Chromosome Replication Research Group, Division of Gene Regulation and Expression, Department of Biochemistry, University of Dundee, Dundee DD1 5EH, UK
During late mitosis and early G1, a series of proteins are assembled onto replication origins, resulting in them becoming ‘licensed’ for replication in the subsequent S phase. Four factors have so far been identified that are required for chromatin to become functionally licensed: ORC (the origin recognition complex) and Cdc6, plus the two components of the replication licensing system RLF-M and RLF-B. Here we describe the first steps of a systematic fractionation of Xenopus egg extracts to identify all the components necessary for the assembly of licensed replication origins on Xenopus sperm nuclei (the physiological DNA substrate in this system). We have purified a new activity essential for this reaction, and have shown that it is nucleoplasmin, a previously known chromatin remodelling protein. Nucleoplasmin decondenses the sperm chromatin by removing protamines, and is required at the earliest known step in origin assembly to allow ORC to bind to the DNA. Sperm nuclei can be licensed by a combination of nucleoplasmin, RLF-M and a partially purified fraction that contains ORC, Cdc6 and RLF-B. This suggests that we are likely to have identified most of the proteins required for this assembly reaction.
* To whom correspondence should be addressed. Tel: +44 1382 345797; Fax: +44 1382 348072; Email: j.j.blow@dundee.ac.uk
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