REVIEWS In this respect, examining the distribution of particular histone variants at particular domains throughout the cell cycle might prove to be highly informative. Dilution of histone variants eplacement with H3.3 Challenges of heterochromatin maintenance during replicat Pericentric heterochromatin domains contribute to cor- rect chromosome segregation and must be maintained ●Q oughout the cell cycle. During mitosis and S phase, the particular molecular marks that characterize pericentric heterochromatin and its higher-order organization(BOX Modification of neighbouring H3. are challenged In different organisms, such as fission yeast and mice. diverse mechanisms have evolved that ensure OHZA-H2B ue身 More stable heterochromatin maintenance -New H3 HH4 T Active mark with H3.3-H4 with H3-H4 Fission yeast Fission yeast spends most of its lifetime in phase, during which pericentric repeats are organ zed into nucleosomes that are enriched in dimethylated H3K9(H3K9me2), to which the HPI homologue Swi6 is Centromeric chromati bound, Swi6 recruits the evolutionarily conserved ring shaped protein complex cohesin, which maintains sister cells enter 1 eeeeee.如油 plicatic 99自自自自 even before cytokinesis is completed o. The dilution of H3. 1 deposition repressive histone marks and further Swi6 delocaliza tion as a consequence of DNA replication are thought H3 eviction to allow access to the RNA polymerase II machiner and the bidirectional transcription of pericentromeric repeats occurs in this discrete cell cycle window of early S phase.I0(FIG5a) New H31H4 Indeed, a careful analysis of transcript levels during new CENP-A depositi the cell cycle reveals a correlation between the timings Figure 4 Inheritance of histone H3 variants outside of S phase. of replication and transcription, as the forward tran al Transcriptionally active domains are enriched in nucleosomes that contain histone scripts that have a transcription start site closer to the H3.3, which have more dynamic conformations and are enriched in active marks 58-1. replication origin accumulate first?. The transcripts Deposition of the histone variant H3. 1 during DNA replication results in the dilution of are processed into small interfering RNAs(siRNAs)that H3.3. Active marks on H3. 3 might recruit factors that facilitate the modification of accumulate transiently in S phase?. RNA interference neighbouring H3. 1 to ensure the inheritance of tive state in a dominant fashion (RNAi)-dependent and RNAi-independent mechanisms Loss of H3. 3 might be counterbalanced by the transcription-dependent incorporation then direct Lys methyltransferase(Clr4; also known function in chromatin assembly independently of DNA synthesis a 93.b incorporation Kmt D)and HDAC activity (Clr3 and Sir2), respectively. of the centromere-specific histone H3 variant CenH3(CENP-A in humans)at to re-establish heterochromatin characteristics follow centromeres is another DNA-synthesis-independent histone-deposition process" ing replication 09-ll. Whereas experimental evidence Replication of centromeric DNA in S phase dilutes CENP-A, resulting in three possible substantiates this model in yeast, whether RNAi is scenarios. First, parental CENP-A is equally distributed to daughter strands as a involved in heterochromatin maintenance in mam- dimer, possibly creating hemisomes. Second, parental CENP-A is distributed onto mals is unclear-ll6. Although pericentric repeats are daughter strands (as either tetramers or dimers)and H3. 1 is temporarily deposited at transcribed, not every component of the fission centromeres,resulting in asymmetric or random distribution. Third, parental CENP-A yeast RNAi machinery, such as RNA-dependent RNA is randomly distributed to daughter strands(as either tetramers or two dimers)and nucleosome 'gaps are created'run, Later in the cell cycle, during late telophase -early amplification of SIRNA production, has been identified specific deposition factors. Eviction of temporary H3. 1 from centromeres might precede in mammals Mice. As in fission yeast, mouse pericentric hetero- chromatin is enriched in HPl proteins, the binding of state that contains one-half of the amount of CENP-A, which is dependent on H3K9me3 as well as an uniden before it is fully replenished with new CENP-A mol- tified structural RNA component20, 12. Although ecules later in the cell cycle. Although the incorporation H3S10 phosphorylation occurs on entry into mitosis of replacement variants H3.3 or CENP-A is not directly in mammals 24123, some HPl is retained during mito dent on DNA replication, the distribution of paren- sis and, in contrast to fission yeast, it is enriched in histones at the fork could potentially pre-determine heterochromatin domains in Gl phase2412.Cell cycle how and when H3. 1 can be replaced at later stages. regulation of the transcription of pericentric repeats 200 MAR 22009 Macmillan Publishers Limited All rights reservedNature Reviews | Molecular Cell Biology Transcription-coupled Dilution of histone v replacement with H3.3 ariants during replication HIRA 1 2 ‘Hemisome’ Temporary H3.1 deposition 3 Late telophase–early G1; new CENP-A deposition ‘Gaps’ Centromeric chromatin H3.3–H4 H2A–H2B ‘Unstable’ nucleosome with H3.3–H4 More stable nucleosome New H3. with H3.1–H4 1–H4 Active mark a b CENP-A–H4 New H3.1–H4 S phase; CENP-A dilution Modification of neighbouring H3.1 X H3.1 eviction or state that contains one­half of the amount of ceNP­A, before it is fully replenished with new ceNP­A mol￾ecules later in the cell cycle. Although the incorporation of replacement variants H3.3 or ceNP­A is not directly dependent on DNA replication, the distribution of paren￾tal histones at the fork could potentially pre­determine how and when H3.1 can be replaced at later stages. In this respect, examining the distribution of particular histone variants at particular domains throughout the cell cycle might prove to be highly informative. Challenges of heterochromatin maintenance Pericentric heterochromatin domains contribute to cor￾rect chromosome segregation and must be maintained throughout the cell cycle. During mitosis and S phase, the particular molecular marks that characterize pericentric heterochromatin and its higher­order organization (BOX 2) are challenged. In different organisms, such as fission yeast and mice, diverse mechanisms have evolved that ensure heterochromatin maintenance. Fission yeast. Fission yeast spends most of its lifetime in G2 phase, during which pericentric repeats are organ￾ized into nucleosomes that are enriched in dimethylated H3K9 (H3K9me2), to which the HP1 homologue Swi6 is bound. Swi6 recruits the evolutionarily conserved ring￾shaped protein complex cohesin, which maintains sister chromatid cohesion103,104. As cells enter mitosis, histone H3 becomes phosphorylated on residue S10, which results in reduced Swi6 binding and facilitates chromo￾some segregation105–107. centromeres undergo replication even before cytokinesis is completed108. The dilution of repressive histone marks and further Swi6 delocaliza￾tion as a consequence of DNA replication are thought to allow access to the RNA polymerase II machinery, and the bidirectional transcription of pericentromeric repeats occurs in this discrete cell cycle window of early S phase106,107 (FIG. 5a). Indeed, a careful analysis of transcript levels during the cell cycle reveals a correlation between the timings of replication and transcription, as the forward tran￾scripts that have a transcription start site closer to the replication origin accumulate first107. The transcripts are processed into small interfering RNAs (siRNAs) that accumulate transiently in S phase107. RNA interference (RNAi)­dependent and RNAi­independent mechanisms then direct lys methyltransferase (clr4; also known as Kmt1) and HDAc activity (clr3 and Sir2), respectively, to re­establish heterochromatin characteristics follow￾ing replication109–113. Whereas experimental evidence substantiates this model in yeast, whether RNAi is involved in heterochromatin maintenance in mam￾mals is unclear114–116. Although pericentric repeats are transcribed117,118, not every component of the fission yeast RNAi machinery, such as RNA­dependent RNA polymerase, which serves in the post­transcriptional amplification of siRNA production, has been identified in mammals119. Mice. As in fission yeast, mouse pericentric hetero￾chromatin is enriched in HP1 proteins, the binding of which is dependent on H3K9me3 as well as an uniden￾tified structural RNA component 120,121. Although H3S10 phosphorylation occurs on entry into mitosis in mammals122,123, some HP1 is retained during mito￾sis and, in contrast to fission yeast, it is enriched in heterochromatin domains in G1 phase124,125. cell cycle regulation of the transcription of pericentric repeats Figure 4 | inheritance of histone H3 variants outside of S phase. a | Transcriptionally active domains are enriched in nucleosomes that contain histone H3.3, which have more dynamic conformations and are enriched in active marks55,89–91. Deposition of the histone variant H3.1 during DNA replication results in the dilution of H3.3. Active marks on H3.3 might recruit factors that facilitate the modification of neighbouring H3.1 to ensure the inheritance of an active state in a dominant fashion. Loss of H3.3 might be counterbalanced by the transcription-dependent incorporation of H3.3 promoted by histone chaperones, such as Hir-related protein A (HIRA), that function in chromatin assembly independently of DNA synthesis59,89,93. b | Incorporation of the centromere-specific histone H3 variant CenH3 (CENP-A in humans) at centromeres is another DNA-synthesis-independent histone-deposition process97. Replication of centromeric DNA in S phase dilutes CENP-A, resulting in three possible scenarios. First, parental CENP-A is equally distributed to daughter strands as a dimer, possibly creating hemisomes. Second, parental CENP-A is distributed onto daughter strands (as either tetramers or dimers) and H3.1 is temporarily deposited at centromeres, resulting in asymmetric or random distribution. Third, parental CENP-A is randomly distributed to daughter strands (as either tetramers or two dimers) and nucleosome ‘gaps’ are created99,101. Later in the cell cycle, during late telophase–early G1 phase, newly synthesized CENP-A is deposited at centromeres98, possibly by specific deposition factors. Eviction of temporary H3.1 from centromeres might precede the deposition of new CENP-A. REVIEWS 200 | mARcH 2009 | VOlume 10 www.nature.com/reviews/molcellbio © 2009 Macmillan Publishers Limited. All rights reserved