Abstract
Centromeres are specialized chromatin regions marked by the presence of nucleosomes containing the centromere-specific histone H3 variant CENP-A, which is essential for chromosome segregation. Assembly and disassembly of nucleosomes is intimately linked to DNA topology, and DNA topoisomerases have previously been implicated in the dynamics of canonical H3 nucleosomes. Here we show that Schizosaccharomyces pombe Top3 and its partner Rqh1 are involved in controlling the levels of CENP-ACnp1 at centromeres. Both top3 and rqh1 mutants display defects in chromosome segregation. Using chromatin immunoprecipitation and tiling microarrays, we show that Top3, unlike Top1 and Top2, is highly enriched at centromeric central domains, demonstrating that Top3 is the major topoisomerase in this region. Moreover, centromeric Top3 occupancy positively correlates with CENP-ACnp1 occupancy. Intriguingly, both top3 and rqh1 mutants display increased relative enrichment of CENP-ACnp1 at centromeric central domains. Thus, Top3 and Rqh1 normally limit the levels of CENP-ACnp1 in this region. This new role is independent of the established function of Top3 and Rqh1 in homologous recombination downstream of Rad51. Therefore, we hypothesize that the Top3-Rqh1 complex has an important role in controlling centromere DNA topology, which in turn affects the dynamics of CENP-ACnp1 nucleosomes.
Highlights
Centromeres are unique regions of eukaryotic chromosomes that are essential for chromosome segregation at mitosis and meiosis
Centromeric chromatin is marked by the presence of nucleosomes containing the histone H3 variant Centromere Protein-A (CENP-A), which is a key determinant for centromere identity and essential for centromere function
Centromeres are unique regions on eukaryotic chromosomes that are essential for chromosome segregation at mitosis and meiosis
Summary
Centromeres are unique regions of eukaryotic chromosomes that are essential for chromosome segregation at mitosis and meiosis. Centromeric chromatin is marked by the presence of nucleosomes containing the histone H3 variant Centromere Protein-A (CENP-A), which is a key determinant for centromere identity and essential for centromere function. During DNA replication in S-phase pre-existing CENP-A is partitioned to sister centromeres and after chromosome segregation newly synthesized CENP-A is incorporated at pre-existing centromeres, possibly involving a feed-forward mechanism between pre-existing CENP-A chromatin and CENP-A assembly factors [1,2]. It binds to pre-nucleosomal CENP-A and histone H4, and has been shown to facilitate assembly of nucleosomes containing CENP-A in vitro [8,9,10,11,12,13,14]. The Mis complex and/or the CENP-A pre-nucleosomal complex associates with the chaperone RbAp48, which can mediate assembly of nucleosomes containing CENP-A in vitro [4,5,10,11,15]
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