Centromere Sequence Organization and Evolution
Centromeres are essential for faithful segregation of genetic material during mitosis and meiosis.
Despite their critical role in cell viability, efforts to define the sequence organization at these chromosomal
sites are challenged by the complex nature of centromeric sequences, leaving these genomic regions incomplete
and largely unexplored. Human centromeres are defined by a predominant tandem repeat family, known as alpha
satellite, resulting in megabase arrays of near sequence identity and limited divergence among many thousands
of copies of repeat units, which are organized in a hierarchical and, in many cases, chromosome-specific manner.
Such sequence complexities, compounded by pooled sequences from diploid individuals, have confounded current
assembly efforts and eliminated the opportunity to distinguish sequence patterns specific to a single haploid
centromere. To address these challenges a novel strategy has been developed to specifically target these
underrepresented sites in the genome and to complete the first comprehensive map of human centromere sequence
organization.
Key Recent Publications
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Schueler MG, Dunn JM, Bird CP, Ross MT, Viggiano L, Rocchi M, Willard HF, Green ED; NISC Comparative Sequencing Program
(2005) Progressive proximal expansion of the primate X chromosome centromere.Proc. Natl. Acad. Sci. USA 102: 10563-10568.
NCBI Link
Breaking the Code of Silence in Fission Yeast
The structure of chromatin in the genome is not uniform.
Single-copy and low-copy number genes that give rise to most cellular mRNAs
are packaged in euchromatin.
In contrast, the heterochromatic fraction of the genome corresponds to
highly condensed chromosomal regions, from which few mRNAs are produced.
Some heterochromatic states are capable of "oozing" across genomic DNA,
thus silencing genes. This creates a unique challenge for the cell: how to regulate
heterochromatin assembly to ensure that appropriate genes are turned off,
while protecting the expression of other genes. Using experimental approaches
that integrate genetics, molecular biology, cytology and genomics, we study the
genomic and epigenetic features that delimit heterochromatin domains in the
fission yeast, Schizosaccharomyces pombe.
Our previous work identified a distinct barrier element, a tRNA gene,
present at the border between pericentromeric heterochromatin and the specialized,
CENP-A containing chromatin at the fission yeast centromere. The tRNA gene is
expressed, despite its centromeric location. Moreover, lack of the barrier leads
to chromosome segregation defects. These initial studies have led us to investigate
the mechanism of barrier activity, as well as its role in chromosome architecture
in mitosis and meiosis. We have also developed an artificial chromosome assay
that will allow us to address how and when the barrier is established and whether
maintenance of distinct chromatin states requires an intact barrier sequence.
To extend our studies beyond the centromere we have created an assay in which sequences
that nucleate heterochromatin assembly have been placed in an otherwise euchromatic region
of the S. pombe genome. The nucleation site establishes a distinct heterochromatin
domain that includes a reporter gene and neighboring endogenous sequences. We are now
poised to ask whether the propagation of heterochromatin depends on primary sequence, trans
acting factors, or a combination of both, and anticipate that our findings will serve as a
model for other examples of epigenetic gene silencing, including X inactivation.
Key Recent Publications
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Scott KC, White C, Willard HF
(2007) An RNA Polymerase III-Dependent Heterochromatin Barrier at Fission Yeast Centromere 1. PLoS ONE. 2: e1099.
NCBI Link
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Scott KC, Merrett SL, Willard HF
(2006) A heterochromatin barrier partitions the fission yeast centromere into discrete chromatin domains. Current Biol. 16: 119-129.
NCBI Link
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