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Department of Cell Biology, The Scripps Research Institute, La Jolla, California 92037
We investigated the requirements for targeting the centromeric histone H3 homologue CENP-A
for assembly at centromeres in human cells by transfection of epitope-tagged CENP-A derivatives into HeLa
cells. Centromeric targeting is driven solely by the conserved histone fold domain of CENP-A. Using the crystal structure of histone H3 as a guide, a series of CENPA/histone H3 chimeras was constructed to test the role
of discrete structural elements of the histone fold domain. Three elements were identified that are necessary for efficient targeting to centromeres. Two correspond to contact sites between histone H3 and
nucleosomal DNA. The third maps to a homotypic
H3-H3 interaction site important for assembly of the
(H3/H4)2 heterotetramer. Immunoprecipitation confirms that CENP-A self-associates in vivo. In addition,
targeting requires that CENP-A expression is uncoupled from histone H3 synthesis during S phase.
CENP-A mRNA accumulates later in the cell cycle than histone H3, peaking in G2. Isolation of the gene
for human CENP-A revealed a regulatory motif in the
promoter region that directs the late S/G2 expression
of other cell cycle-dependent transcripts such as cdc2,
cdc25C, and cyclin A. Our data suggest a mechanism
for molecular recognition of centromeric DNA at the nucleosomal level mediated by a cooperative series of
differentiated CENP-A-DNA contact sites arrayed
across the surface of a CENP-A nucleosome and a
distinctive assembly pathway occurring late in the cell
cycle.
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