Establishment and maintenance of alternative chromatin states at a multicopy gene locus.

In eukaryotes, each of the more than 100 copies of ribosomal RNA (rRNA) genes exists in either an RNA polymerase I transcribed open chromatin state or a nucleosomal, closed chromatin state. Open rRNA genes guarantee the cell's supply with structural components of the ribosome, whereas closed rRNA genes ensure genomic integrity. We report that the observed balance between open and closed rRNA gene chromatin states in proliferating yeast cells is due to a dynamic equilibrium of transcription-dependent removal and replication-dependent assembly of nucleosomes. Pol I transcription is required for the association of the HMG box protein Hmo1 with open rRNA genes, counteracting replication-independent nucleosome deposition and maintaining the open rRNA gene chromatin state outside of S phase. The findings indicate that the opposing effects of replication and transcription lead to a de novo establishment of chromatin states for rRNA genes during each cell cycle.