CHAPTER
Regulation of Gene Expression
The number of molecules of a protein produced per unit
time from a particular gene differs from gene to gene,
sometimes greatly. This variation can occur in several
ways. For some genes, the strength of a promoter or a
ribosome binding site is sufficient to regulate the required
concentration of a particular protein. However, other gene
products are needed only occasionally, e.g., when a par-
ticular nutrient is present in the surrounding medium; still
other gene products are required only during particular
stages of differentiation or in specific cell types. For such
genes, the products are usually not present in significant
amounts except when gene expression is activated, and
the level of gene expression is determined by an on-off
switch of some sort. In this chapter, numerous mechanisms
are described by which differences in gene expression are
achieved.
26.1
Regulation of mRNA
Regulation of gene expression is achieved by
1. Regulation of the number of mRNA molecules
produced per unit time,
2. Regulation of the translation of mRNA,
3. Regulation of the number of copies of a gene, and
4. Posttranslational modification.
In prokaryotes, mRNA synthesis can be controlled
simply by regulating initiation of transcription. In eu-
karyotes, mRNA is formed from a primary transcript
followed by a series of processing events (e.g., intron ex-
cision, polyadenylylation). Eukaryotes regulate not only
transcription initiation but the various stages of processing
as well.
An important aspect of
mRNA regulation
is determined
by the turnover of mRNA molecules, i.e., translation can
occur only as long as the mRNA remains intact. In bacteria,
mRNA molecules have a lifetime of only a few minutes,
and continued synthesis of mRNA molecules is needed to
maintain synthesis of the proteins encoded in the mRNAs.
In eukaryotes, the lifetime of mRNA is generally quite
long (hours or days), thereby enabling a small number of
transcription initiation events to produce proteins over a
long period of time.
Metabolic pathways normally consist of a large num-
ber of enzymes; in some cases, the individual enzymes
are used in a particular pathway and nowhere else. In
these cases, it is efficient to regulate expression of all or
none of the enzymes. In bacterial systems, all enzymes of
the pathway are encoded in a single polycistronic mRNA
molecule, and synthesis of the mRNA produces all the en-
zymes. In eukaryotes, common signals for transcription
of different genes may be used or the primary transcript
can be differentially processed to yield a set of mRNA
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