REPLICATION OF TRANSCRIPTIONALLY ACTIVE CHROMATIN

IN eukaryotic cells, active genes and their regulatory sequences are organized into open chromatin conformations in which nucleosomes can be modified, disrupted or totally absent(1-3). It has been proposed that these characteristic chromatin structures and their associated factors might be directly inherited by the newly synthesized daughter strands during chromosome duplication(4-6). Here we show that in the yeast Saccharomyces cerevisiae, replication machinery entering upstream of a transcriptionally active ribosomal RNA gene generates two newly replicated coding regions regularly packaged into nucleosomes, indicating that the active chromatin structure cannot be directly inherited at the replication fork. Whereas the establishment of an exposed chromatin conformation at some newly replicated rRNA gene promoters can occur shortly after the passage of the replication fork, regeneration of the active chromatin structure along the coding region is always a post-replicative process involving disruption of preformed nucleosomes.