The nucleosome, which is composed of DNA wrapped around a histone octamer, is a 
fundamental unit of chromatin and is duplicated during the eukaryotic DNA 
replication process. The evolutionarily conserved histone chaperone cell cycle 
gene 1 (CCG1) interacting factor A/anti-silencing function 1 (CIA/Asf1) is 
involved in histone transfer and nucleosome reassembly during DNA replication. 
CIA/Asf1 has been reported to split the histone (H3-H4)(2) tetramer into histone 
H3-H4 dimer(s) in vitro, raising a possibility that, in DNA replication, 
CIA/Asf1 is involved in nucleosome disassembly and the promotion of 
semi-conservative histone H3-H4 dimer deposition onto each daughter strand in 
vivo. Despite numerous studies on the functional roles of CIA/Asf1, its 
mechanistic role(s) remains elusive because of lack of biochemical analyses. The 
biochemical studies described here show that a V94R CIA/Asf1 mutant, which lacks 
histone (H3-H4)(2) tetramer splitting activity, does not form efficiently a 
quaternary complex with histones H3-H4 and the minichromosome maintenance 2 
(Mcm2) subunit of the Mcm2-7 replicative DNA helicase. Interestingly, the mutant 
enhances nascent DNA strand synthesis in a cell-free chromosomal DNA replication 
system using Xenopus egg extracts. These results suggest that CIA/Asf1 in the 
CIA/Asf1-H3-H4-Mcm2 complex, which is considered to be an intermediate in 
histone transfer during DNA replication, negatively regulates the progression of 
the replication fork.