Replication foci dynamics: replication patterns are modulated by S-phase checkpoint kinases in fission yeast.

Although the molecular enzymology of DNA replication is well characterised, how and why it occurs in discrete nuclear foci is unclear. Using fission yeast, we show that replication takes place in a limited number of replication foci, whose distribution changes with progression through S phase. These sites define replication factories which contain on average 14 replication forks. We show for the first time that entire foci are mobile, able both to fuse and re-segregate. These foci form distinguishable patterns during S phase, whose succession is reproducible, defining early-, mid- and late-S phase. In wild-type cells, this same temporal sequence can be detected in the presence of hydroxyurea (HU), despite the reduced rate of replication. In cells lacking the intra-S checkpoint kinase Cds1, replication factories dismantle on HU. Intriguingly, even in the absence of DNA damage, the replication foci in cds1 cells assume a novel distribution that is not present in wild-type cells, arguing that Cds1 kinase activity contributes to the spatio-temporal organisation of replication during normal cell growth.