Eukaryotic chromosomal DNA replication requires a two-step assembly of replication proteins on origins; formation of the prereplicative complex (pre-RC) in late M and G1 phases of the cell cycle, and assembly of other replication proteins in S phase to load DNA polymerases to initiate DNA synthesis. In budding yeast, assembly of Dpb11 and the Sld3-Cdc45 complex on the pre-RC at origins is required for loading DNA polymerases. Here we describe a novel replication complex, GINS (Go, Ichi, Nii, and San; five, one, two, and three in Japanese), in budding yeast, consisting of Sld5, Psf1 (partner of Sld five 1), Psf2, and Psf3 proteins, all of which are highly conserved in eukaryotic cells. Since the conditional mutations of Sld5 and Psf1 confer defect of DNA replication under nonpermissive conditions, GINS is suggested to function for chromosomal DNA replication. Consistently, in S phase, GINS associates first with replication origins and then with neighboring sequences. Without GINS, neither Dpb11 nor Cdc45 associates properly with chromatin DNA. Conversely, without Dpb11 or Sld3, GINS does not associate with origins. Moreover, genetic and two-hybrid interactions suggest that GINS interacts with Sld3 and Dpb11. Therefore, Dpb11, Sld3, Cdc45, and GINS assemble in a mutually dependent manner on replication origins to initiate DNA synthesis.[Keywords: DNA replication; replication protein; Dpb11; Sld3; Sld5] Supplemental material is available at http://www.genesdev.org.
