Page 35 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

P1: GNH Revised Pages
Encyclopedia of Physical Science and Technology EN002G-104 May 17, 2001 20:53

Chromatin Structure and Modification 827

established its HAT activity as essential to transcriptional ment and skeletal maturation (thyroid hormone), and oth-
activation. For example, work from Allis and coworkers ers.Pointmutationsinthegeneforthyroidhormonerecep-
showed that GCN5 is required for the transcriptional up- tor(TR)leadtotheclinicaldisorderofresistancetothyroid
regulation of a number of budding yeast genes, and that hormone—patients present with goiter, stunted growth,
point mutations in Gcn5p that abrogate its catalytic activ- and attention deficit hyperactivity disorder. Biochemical
ity as a HAT abolish its capacity to abet transcriptional analysis in V. K. Chatterjee’s lab revealed that the muta-
control. In addition, ChIP analysis demonstrated that his- tions impair the capacity of TR to recruit HAT coactivators
tones over promoters of genes that are targets for bind- and lower—or abolish, in some cases—its ability to acti-
ing by Gcn5p-recruiting activators are hyperacetylated vate transcription. These observations present very strong
concomitant with its action. Finally, the laboratory of S. evidence that HAT targeting by specific transcriptional ac-
Roth reported an experiment that is the scientific equiv- tivators occurs in vivo and is relevant to genomic control.
alent of a coup de grace: a yeast strain was engineered The reaction opposite to that effected by HATs is cat-
in which specific lysines in histone tails were mutated alyzed by histone deacetylases (HDACs). The history of
to a noncharged residue—thus, these cells had “geneti- their discovery begins, once again, in the mid-1980s, when
cally hyperacetylated histones.” In such a strain, formerly a screen in budding yeast performed in the laboratory of
GCN5-dependent genes lost their requirement for Gcn5p R. Gaber identified a number of loci that reverse a potas-
to become transcriptionally active!—thus, it was formally sium transport deficiency (=RPD) by virtue of the fact that
proven that the in vivo function of Gcn5p at target gene mutations in those loci lead to the upregulation of potas-
promoters is to hyperacetylate the histones. sium transporters. One such locus, RPD3, was subjected
While original analysis revealed a single major HAT in to additional genetic analysis and found to be involved in
Tetrahymena extracts, subsequent work revealed the fact the control of transcription of a number of yeast genes.
that eukaryotic genomes contain a large number of pro- In 1996, the laboratory of S. Schreiber used an ingenious
teinspossessingHATactivity.Importantly,anoverwhelm- purification strategy to isolate a mammalian HDAC: these
ing majority of these can interact with various transcrip- researchers reasoned that a competitive inhibitor could be
tion activators (hence the term “coactivator” that is used to used as bait for an HDAC (such a compound binds to the
describe the HATs). In metazoa, the most-studied HAT is catalytic site of an enzyme but prevents catalysis due to
the global transcriptional coactivator p300 and the closely its structural difference from the enzyme’s bona fide sub-
related protein CBP; this large protein (ca. 2400 amino strate). Thus, the peptide trapoxin was used to prepare
acids) contains at least four distinct interaction interfaces an affinity matrix through which crude mammalian cell
that allow it to be targeted by an extraordinary variety extract was passed; two polypeptides were found to asso-
of transcriptional regulators, including proteins that reg- ciate with the matrix—and peptide microsequence analy-
ulate the cell cycle (such as c-jun and c-Fos), cell differ- sis of one of them revealed its close sequence similarity to
entiation (MyoD), cell-cycle checkpoints (p53), and the budding yeast Rpd3p. Because activity assays indicated
nuclear hormone receptors. Most importantly, as discov- this newly purified protein to have HDAC activity, these
ered in the laboratory of Y. Nakatani in 1996, from an data were accepted as the first evidence that an HDAC
enzymatic standpoint CBP is a HAT capable of hyper- in mammals may be involved in transcriptional control.
acetylating all four core histones in solution. The ubiquity Subsequent analysis of mammalian RPD3 (also known as
of CBP/p300’s involvement in transcriptional regulatory HDAC1) confirmed that notion.
pathways in vivo is powerful evidence to the pervasive use A dedicated effort to clone additional HDACs from eu-
of targeted chromatin remodeling to effect gene control. In karyotic genomes revealed at least eight distinct genes—it
humans, mutations in the gene for CBP cause Rubinstein– isverylikelythatmorewillbeidentifiedoncetheemerging
Taybi syndrome—a multisymptomatic disorder character- sequences of metazoan genomes, nematode, insect, rho-
ized by mental retardation and a complex pattern of pro- dent, and primate, are analyzed in sufficient detail. Several
found developmental abnormalities. general observations can be made at this time, however.
A well-characterized group of transcription factors that From a functional standpoint, many of the HDACs have
use HAT targeting to effect transcriptional activation are been functionally connected to transcriptional repression
the nuclear hormone receptors—these recruit such HATs pathways; two examples are informative.
as SRC-1 and ACTR. Thus, HATs are an integral compo- The first one involves one of the strongest mechanisms
nent of signal transduction pathways involving major reg- for effecting transcriptional repression currently known:
ulators of mammalian physiology—glucose metabolism DNA methylation. Genomes of higher vertebrates con-
(glucocorticoids), ovulation (progesterone), development tain unusually low amounts of the dinucleotide CpG (the-
of secondary sexual characteristics (estradiol and testos- oretical predictions suggest each dinucleotide should ac-
2
terone), bone morphogenesis (vitamin D), brain develop- count for 1/4 = 0.0625, i.e., ca. 6% of the genome; CpG