REPLICATION OF TRANSCRIPTIONALLY ACTIVE CHROMATIN

IN eukaryotic cells, active genes and their regulatory sequences are o rganized 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 factor s might be directly inherited by the newly synthesized daughter strand s during chromosome duplication(4-6). Here we show that in the yeast S accharomyces cerevisiae, replication machinery entering upstream of a transcriptionally active ribosomal RNA gene generates two newly replic ated coding regions regularly packaged into nucleosomes, indicating th at the active chromatin structure cannot be directly inherited at the replication fork. Whereas the establishment of an exposed chromatin co nformation at some newly replicated rRNA gene promoters can occur shor tly after the passage of the replication fork, regeneration of the act ive chromatin structure along the coding region is always a post-repli cative process involving disruption of preformed nucleosomes.