can simultaneously bind DNA and other transcription factors or RNAP II to regulate gene
transcription (Fig. 6.8).
6.3.3 Coordinated Regulation of Gene Expression
Eukaryotes can coordinately express subsets of many different genes in response to particu-
lar biotic and abiotic signals because those genes will contain commoncis-regulatory or
response elements in their promoters or enhancers that allow them to recognize the same
signals. These elements have a consensus sequence that can bind specific transcription
factors allowing for transcription of those genes. A gene may also contain several different
response elements allowing it to be expressed following a number of stimuli. For example,
the CBF transcription factors ofArabidopsis(Gilmour et al. 1998) can bind to the cold- and
dehydration-responsivecis-regulatory element called CRT/DRE (C-repeat/dehydration-
responsive element) (Baker et al. 1994; Yamaguchi-Shinozaki and Shinozaki 1994) that
is found in the promoters of many cold- and dehydration-responsive genes of
Arabidopsis. So, following cold or water-stress stimuli, those genes containing the CRT/
DRE responsive element will be transcribed and provideArabidopsiswith increased toler-
ance to freezing, as well as drought.
6.3.4 Chromatin as an Important Regulator of Transcription
DNA wrapped around histones and coiled to produce chromatin is not accessible for tran-
scription. It is not physically possible for RNAP II to make contact with the DNA for
Figure 6.8.Transcription factors structure and function. Transcription factors may have domains that
bindcis-acting elements such as enhancers, and domains that also bindtrans-acting elements such as
RNA polymerase (RNAP II) and other transcription factors. See color insert.
144 MOLECULAR GENETICS OF GENE EXPRESSION