Control of Eukaryotic Gene Expression
Campbell Chap. 19.
Ed. pp. 354-356 & 362-367 (top)
Ed. Chap 17 pp. 359-367
Ed. 320-322, 356-362
In any organism, only a subset of genes is expressed and protein products made at any time.
cells can adjust the complement of proteins they make to reflect their specific situation - for
instance, their nutritional status.
We saw an example of this in the Trp operon in bacteria, which is
only expressed when Trp levels are low and the organism needs to make more.
Eukaryotic cells can
also regulate gene expression to reflect particular needs at a given time.
eukaryotes also have an additional type of regulation.
Some cells in these organisms make certain
specialized proteins that other cells never need to make.
That is, some genes are never expressed in
most cells in the body, but are only expressed in certain tissues.
The amount of a particular protein found in a cell at any time, and the ability of the protein to
function, can be regulated at any of several levels (See Fig 19.3, 7
ed.; 18.6, 8
include rate of transcription of the gene, RNA processing & transport to the cytoplasm, rate of
translation of the mRNA, rate of mRNA degradation, modification of the protein, rate of protein
degradation, and transport to the appropriate site in the cell.
We already saw an example of how the
levels of cyclin proteins are regulated through the cell cycle by protein degradation.
We’ll see an
example later in the course of how transport to the right place regulates protein function.
findings on micro RNA suggest that mRNA degradation plays a more important role in regulating
protein levels than we’d realized.
However, in most cases, the major factor determining the amount
of a protein in the cell is its level of transcription.
Generally, if an mRNA is made, it will be
translated into functional protein.
That is, the amount of a functional protein present in cells is
usually proportional to the amount of its mRNA. For this reason, we’ll focus on learning how
transcription rate is regulated.
Part I: Role of Transcription Factors in Regulating Gene Expression
Introduction: Compare with bacteria
Remember that in the Trp operon in bacteria, the Trp
repressor (when it’s bound to Trp) binds the operator to block transcription by RNA polymerase.
Here are some differences between this situation and how gene regulation works in eukaryotes.
(eukaryotes) have no operons; each of our genes has its own promoter.
In addition, we don’t have
repressor proteins that act by binding to operators and physically blocking RNA polymerase.
we don’t have operator sequences next to our genes).
In addition, in contrast to bacteria, in
eukaryotes it’s relatively rare to have small molecules bind regulatory proteins, and for this binding
to determine whether or not the regulatory proteins bind DNA (as was the case for binding of Trp to
the Trp repressor in E. coli).