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Summary: trends in CELL BIOLOGY (Vol. 9) May 1999 0962-8924/99/$ see front matter © 1999 Elsevier Science. All rights reserved. 193
PII: S0962-8924(99)01536-6
The nucleosome consists of 146 base pairs of DNA
wrapped twice around an octamer that comprises
two copies of each of the four core histone proteins
H2A, H2B, H3 and H4. Nucleosomes are assembled
onto DNA in repeating arrays to yield the familiar
beads-on-string structure of chromatin. The assem-
bly of DNA into nucleosomes or higher-order
chromatin structures renders the underlying DNA
template transcriptionally inactive by blocking ac-
cess to regulatory proteins and the transcriptional
machinery. Elaborate mechanisms exist both to
circumvent and to establish these repressive chro-
matin structures (Fig. 1). One mechanism that has
received a great deal of attention recently is the re-
versible acetylation of the N-terminal tails of the
core histones. In general, regions of chromatin that
are hyperacetylated are transcriptionally active,
whereas regions that are hypoacetylated are inac-
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