The controlling sites in eukaryotes are similar to those found in bacteria,
however there are many more controlling sites and proteins affecting
each eukaryotic gene. Transcription factors (TFs) attach to binding
sites in promoter regions and stimulate RNA polymerase binding to the
promoter site (Figure 4-3a,b). Transcription factors are produced constitutively
since great numbers of genes depend on these for expression.
 |
| Figure 4-3 Transcriptional activation in eukaryotes. |
Enhancers are bound by transcription activators (TAs) that are synthesized
in response to specific signals. Most enhancers, such as the one that
binds Gal4, are located hundreds or thousands of base pairs from the promoter
sites. However, some induced activators, such as Fos and Jun, bind
very near promoter sites. TAs cause the DNA to loop back on itself when
they interact with the TFs near the promoter. This interaction between enhancer
sites and initiator sites is usually necessary for transcription above
a basal level (Figure 4-3c). Coactivators are activator proteins that often
connect TFs and/or TAs with the RNA polymerase and may be essential
for gene expression (Figure 4-3d).
Whereas only one RNApolymerase functions in bacterial cells, three
different RNApolymerases are involved in eukaryotic nuclear transcription. The three polymerases initiate transcription only with specific combinations
of TFs and TAs. RNA polymerase I transcribes the genes that
specify 5.8S, 28S, and 18S rRNA. This polymerase is often found associated
with chromosomes in the nucleoli. RNA polymerase II transcribes
from promoters that control the synthesis of pre-mRNA, which
consists of coding (exons) and noncoding (introns) regions. RNA polymerase
III recognizes promoters that control the synthesis of relatively
short RNAs such as tRNAs, 5S rRNA, and others.
In summary:
RNA pol I → rRNA
RNA pol II → mRNA
RNA pol III → tRNA (and 5S rRNA)
The binding of RNApolymerase to promoter sites is dependent on a
number of TFs such as the TFIID complex (Figure 4-3a), which is
functionally comparable to sigma factors in bacteria. TFIID is the first to
bind close to the promoter at a site called a TATA box or Hogness box
about -20 to -40 bp before the transcription start site. Once bound,
TFIID helps organize other TFs required for initiation of RNA synthesis
(Figure 4-3b). The complex of transcription factors and RNA polymerase
comprise the preinitiation complex, which yields basal levels of
transcription. Induction to higher levels requires the presence of other activators
binding to enhancer elements. Activator proteins like Ctf, Sp1,
and Oct-1 (Figure 4-3c) cause the DNA to loop back upon itself so
that the activator proteins interact with the preinitiation complex, signalling
the RNA polymerase to begin synthesizing high levels of RNA.