transcription. Chromatin remodeling, as mentioned earlier, is required to allow the app-
ropriate regions of a gene to bind transcription factors and RNAP II for transcription.
This remodeling “opens up” the DNA to make it accessible to RNAP II and transcription
factors. Following remodeling, the promoter region no longer contains histones, thereby
making thecis-regulatory elements free to bind to the necessary transcription factors and
RNAP II to begin transcription. Chromatin remodeling is done by various multiprotein
complexes that have ATP-ase activity (use energy) to bind directly to particular regions
of the DNA to move the nucleosomes to a new position to expose the DNA for transcription
(Vignali et al. 2000).
Chromatin structure also can be changed through the covalent addition of acetyl groups
(CH 3 CO) to the histones of the nucleosome. When the acetyl groups are added to the
histone tails, they are no longer positively charged and consequently the negatively
charged DNA can disengage from them. The acetyl groups are added by enzymes called
histone acetyltransferases(HATs). It is known that certain transcription factors have acetyl-
transferase activity or can recruit these enzymes to the DNA, thereby altering chromatin
structure and allowing for transcription. Chromatin structure can be restored by histone dea-
cetylase complexes (HDACs) that remove the acetyl groups from these histones. A good
example of such regulation of gene expression can be found in the control of flowering
inArabidopsis. If the geneflowering locus C(FLC) is expressed, flowering does not
occur. However, if theflowering locus D(FLD) gene is active, it produces a deacetylase
that removes acetyls from histones aroundFLC. Consequently, transcription ofFLC
cannot occur owing to the restoration of chromatin structure, and silencing of FLC
allows flowering (Fig. 6.9) (He et al. 2003).
Figure 6.9.Control of transcription by chromatin remodeling. Genetic regulation of flowering in
Arabidopsis.If the geneflowering locus C(FLC) is expressed, flowering does not occur. However,
if theflowering locus D(FLD) gene is active, it produces a deacetylase that removes acetyls from his-
tones aroundFLC. Consequently, transcription ofFLCcannot occur because of the restoration of
chromatin structure, and silencing ofFLCallows flowering.
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