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DNA replication is essential for cell proliferation, and the replication research has been one of the traditional areas in molecular biology. The basic principal of semi-conservative DNA replication is conserved in all organisms including prokaryotes and eukaryotes, but eukaryotic DNA replication is under the more complicated control in the cell cycle. I have been interested in and studying how replication fork is formed at the onset of S-phase and how fork progression is regulated and ensured under stressful conditions such as DNA damage using Xenopus egg extracts, which can recapitulate semi-conservative DNA replication as well as DNA checkpoint and DNA damage response in vitro.

In the first part, I will present our previous studies about TopBP1 together with the current progress in the field. It was known that S-phase CDK (S-CDK) is required for replication initiation, but the target has been long unknown. In this study, we demonstrated that Xenopus TopBP1 is required for the action of S-CDK in the initiation of DNA replication (Hashimoto and Takisawa, EMBO J. 2003). In addition, TopBP1 was also required for the activation of ATR-Chk1 pathway, a core signalling reaction in the replication checkpoint (Hashimoto et al, Genes Cells 2006).

In the second part, I will present my recent studies about the role of Rad51. Replication block stimulates recombination, but it has been poorly understood how recombination is involved in replication forks in eukaryotes. Therefore I examined the role of Rad51, a central recombination factor, during DNA replication. The analysis of replication intermediates by electron microscopy demonstrated that Rad51 prevents single strand DNA gap accumulation at and behind replication forks in two distinctive manners (Hashimoto et al, Nat. Struct. Mol. Biol. 2010).