Biology Reference
In-Depth Information
Mediator
General TFs
Transcription
mRNA
RNA
polymerase II
Transcription
factor
ORF
UAS
TATA
Figure 2.1. Molecular architecture of regulated gene expression controlled by RNA polymerase II.
The open reading frame (ORF) of a gene is transcribed into mRNA by a combination of
RNA polymerase II, general transcription factors (TFs), a Mediator complex, and
specific transcription factors that bind to particular DNA sequences present in the
gene's promoter region. RNA polymerase with the general TFs and the Mediator
complex can bind to TATA box motifs to stimulate unregulated, constitutive expres-
sion. But for specific, regulated gene expression additional transcription factors bind to
the promoter. These contact upstream activating sequences (UAS) in the promoter and
upon stimulation activate RNA polymerase II through the Mediator complex. Thus, a
combination of specific transcription factors and RNA polymerase together controls
regulated gene expression.
various transcription factors bind that control the genes' expression. The first of
these motifs is the TATA box, to which RNA polymerase II binds, the enzyme
that directly converts the gene to mRNA during transcription. For some genes,
binding of RNA polymerase II along with general transcription factors and the
Mediator complex to the promoter is sufficient to allow continuous, basal
transcription to occur. But for controlled, regulated gene expression, a more
complex promoter structure is observed. Instead, further DNAmotifs are present,
usually upstream of the TATA box, to which bind additional transcription
factors which specifically regulate the activity of RNA polymerase II through
the Mediator complex, to ensure that gene expression only occurs at the required
time, or in the correct cell type. These extra motifs are known collectively in
yeasts as upstream activating sequences (UASs), and enhancers in humans.
Many different UASs have been identified, and in each case the UAS is bound
by a particular transcription factor or complex, which modulates the activity of
RNA polymerase II and consequent gene expression.
As will be seen in this review, in the context of cell cycle regulated gene
expression in yeasts, a large number of UASs have been described, and the
specific cell cycle transcription factor complexes that bind to them. It is the
combination of these two that results in gene expression in these organisms at
certain cell cycle times. Therefore, a primary aim of this review is to describe the
various UASs and transcription factor complexes that control regulated, cell
cycle-specific gene expression in yeasts.
