The unconventional structure of centromeric nucleosomes

Steven Henikoff,Takehito Furuyama

doi:10.1007/s00412-012-0372-y

Abstract

The centromere is a defining feature of the eukaryotic chromosome, required for attachment to spindle microtubules and segregation to the poles at both mitosis and meiosis. The fundamental unit of centromere identity is the centromere-specific nucleosome, in which the centromeric histone 3 (cenH3) variant takes the place of H3. The structure of the cenH3 nucleosome has been the subject of controversy, as mutually exclusive models have been proposed, including conventional and unconventional left-handed octamers (octasomes), hexamers with non-histone protein constituents, and right-handed heterotypic tetramers (hemisomes). Hemisomes have been isolated from native centromeric chromatin, but traditional nucleosome assembly protocols have generally yielded partially unwrapped left-handed octameric nucleosomes. In budding yeast, topology analysis and high-resolution mapping has revealed that a single right-handed cenH3 hemisome occupies the ~80-bp Centromere DNA Element II (CDEII) of each chromosome. Overproduction of cenH3 leads to promiscuous low-level incorporation of octasome-sized particles throughout the yeast genome. We propose that the right-handed cenH3 hemisome is the universal unit of centromeric chromatin, and that the inherent instability of partially unwrapped left-handed cenH3 octamers is an adaptation to prevent formation of neocentromeres on chromosome arms.

Highlights

The centromere is the most familiar of chromosomal landmarks, having been described by 19th century cell biologists (Flemming 1882)
Single base-pair resolution mapping of centromeric chromatin was later obtained using micrococcal nuclease (MNase) digestion followed by paired-end sequencing of nucleosome-sized particles, which showed that occupancy is precisely delimited to the Centromere DNA Element (CDE) (Cole et al 2011)
Tripartite centromeric particles were flanked by well-phased nucleosomes on either side, with subnucleosomal particles occupying short linker regions in between (Fig. 3). This mapping of the Cse4 nucleosome to the ~80-bp CDE of all 16 yeast centromeres implies that there is only a single DNA wrap around the particle, which is inconsistent with an octasome, but it does not distinguish between Cse4/H4/H2B/ H2A hemisomes and (Cse4/H4)2 tetrasomes

Summary

Introduction

The centromere is the most familiar of chromosomal landmarks, having been described by 19th century cell biologists (Flemming 1882). A report published in 2007 challenged this assumption with evidence suggesting that cenH3 (Cse4) nucleosomes of budding yeast lack H2A/H2B dimers and instead package DNA with a core particle containing two copies of a non-histone protein, Scm3 (Mizuguchi et al 2007; Xiao et al 2011).

Results

Conclusion