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Vol. 10, Issue 12, 4217-4230, December 1999
Department of Cell and Molecular Biology, Northwestern University
Medical School, Chicago, Illinois 60611-3008
In Euplotes crassus, most of the micronuclear genome
is eliminated during formation of a transcriptionally active
macronucleus. To understand how this is mediated throughout the genome,
we have examined the chromatin structure of the macronucleus-destined sequences and Tec transposons, which are dispersed in 15,000 copies in
the micronuclear genome and completely eliminated during formation of
the macronuclear genome. Whereas the macronucleus-destined sequences
show a typical pattern of nucleosomal repeats in micrococcal nuclease
digests, the Tec element chromatin structure digests to a
nucleosome-like repeat pattern that is not typical: the minimum digestion products are ~300-600 base pairs, or "subnucleosomal," in size. In addition, the excised, circular forms of the Tec elements are exceedingly resistant to nucleases. Nevertheless, an underlying nucleosomal structure of the Tec elements can be demonstrated from the
size differences between repeats in partial micrococcal nuclease
digests and by trypsin treatment of nuclei, which results in
mononucleosome-sized products. Characterization of the most micrococcal
nuclease-resistant DNA indicates that micronuclear telomeres are
organized into a chromatin structure with digestion properties
identical to those of the Tec elements in the developing macronucleus.
Thus, these major repetitive sequence components of the micronuclear
genome differ in their chromatin structure from the
macronuclear-destined sequences during DNA elimination. The potential
role of developmental stage-specific histone variants in this
chromatin differentiation is discussed.
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