Histone supply and demand are tightly coupled at the replication fork by a chaperone, according to Genevieve Almouzni (Curie Institute, Paris, France) and colleagues. The chaperone, Asf1, passes histones from parental to daughter DNA strands and unwinds just enough DNA to match its transfer activities. Figure 1 Helicase-induced ssDNA (green) is reduced and replication is halted by depletion of a histone chaperone (bottom). While investigating histone dynamics during replication, the authors found Asf1 in complex with a helicase and two histones. When the group knocked down Asf1, the cells stalled in S phase. Finding no defect in replication initiation and no activated checkpoint to halt replication, the authors suspected that the absence of Asf1 decreased the activity of the helicase, which must first unwind DNA for it to be replicated. They found very little helicase-formed ssDNA at replication sites in cells lacking Asf1, indicating that helicase progression was disabled. With functional Asf1, but with replication halted by a chemical inhibitor, the number of Asf1/histone/helicase complexes increased. The authors concluded that Asf1 was picking up histones from the parental DNA strand but was unable to offload them in the absence of new daughter strand synthesis. Replication was also stopped by flooding the cell with histones: since Asf1 was loaded with the new histones, it could not remove any more from the parental DNA strand, thus stopping unwinding. “This system coordinates replication with the packaging of DNA into chromatin,” Almouzni says. It also ensures that the charged and sticky histones are chaperoned and prevents DNA from unwinding in the absence of replication. The movements of histone-laden Asf1 during replication are still unclear. Given the large distances involved, Almouzni envisages Asf1 shuttling back and forth from parent to daughter strands, rather than straddling the gap. Reference: Groth, A., et al. 2007. Science. 318:1928–1931. [PubMed]