Transcription and FACT facilitate the restoration of replication-coupled chromatin assembly defects

Genome duplication occurs through the coordinated action of DNA replication and nucleosome assembly at replication forks. Defective nucleosome assembly causes DNA lesions by fork breakage that need to be repaired. In addition, it causes a loss of chromatin integrity. These chromatin alterations can be restored, even though the mechanisms are unknown. Here, we show that the process of chromatin restoration can deal with highly severe chromatin defects induced by the absence of the chaperones CAF1 and Rtt106 or a strong reduction in the pool of available histones, and that this process can be followed by analyzing the topoisomer distribution of the 2µ plasmid. Using this assay, we demonstrate that chromatin restoration is slow and independent of checkpoint activation, whereas it requires the action of transcription and the FACT complex. Therefore, cells are able to "repair" not only DNA lesions but also chromatin alterations associated with defective nucleosome assembly.