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chapter 26
Regulation of Gene Expression
Polyproteins
In prokaryotes, coordinated regulation of the synthesis of
several gene products is accomplished by regulation of the
synthesis of a single polycistronic mRNA molecule encod-
ing all of the products. The analogue to this arrangement
in eukaryotes is the synthesis of a
polyprotein,
a large
polypeptide that is cleaved after translation to yield in-
dividual proteins. Each protein can be thought of as the
product of a single gene. In such a system, the coding
sequences of each gene in the polyprotein unit are not
separated by stop and start codons but instead by specific
amino acid sequences that are recognized as cleavage sites
by particular protein-cutting enzymes. Polyproteins have
been observed with up to eight cleavage sites; the cleav-
age sites are cut not simultaneously but rather in a specific
order. Use of a polyprotein serves to maintain an equal
molar ratio of the constituent proteins; moreover, delay in
cutting at certain sites introduces a temporal sequence of
production of individual proteins, a mechanism frequently
used by animal viruses.
Some polyproteins are differentially cleaved in different
tissues. An example is
proopiomelanocortin,
a polypro-
tein that is the source of several hormones synthesized
in the pituitary gland. In the anterior lobe of the pitu-
itary, the polyprotein is cleaved to release /3-lipotropin and
adrenocorticotropic hormone (ACTH). In the intermediate
lobe, a different pattern of cleavage forms /
1
-endorphin and
a-melanotropin (Chapter 31).
Translational Regulation
Translational regulation
refers to the number of times a
finished mRNA molecule is translated. The three ways in
which translation of a particular mRNA may be regulated
are
1. By the lifetime of the mRNA,
2. By the probability of initiation of translation, and
3. By regulation of the rate of overall protein synthesis.
The silk gland of the silkworm
Bombyx mori
predom-
inantly synthesizes a single type of protein,
silk fibroin.
Since the worm takes several days to construct its cocoon,
it is the amount and not the rate of fibroin synthesis that
must be great; hence, the silkworm can manage with a fi-
broin mRNA molecule that is very long lived. Silk produc-
tion begins with chromatid amplification in which, over a
period of several days, the single cell of which the gland
is composed has a ploidy of 106. Each fibroin gene is
transcribed from a strong promoter to yield about
1 0 4
fi-
broin mRNA molecules. An “average” eukaryotic mRNA
molecule has a lifetime of 3 hours before it is degraded.
However, fibroin mRNA survives for several days, during
which each mRNA molecule is translated repeatedly to
yield 10
5
fibroin molecules. Thus, the whole unicellular
silk gland makes 10
15
molecules or 300 mg of fibroin.
Production of a large amount of a single type of pro-
tein by means of a prolonged mRNA lifetime is common
in highly differentiated cells. For example, cells of the
chicken oviduct, which makes ovalbumin (for egg white),
contain only a single copy of the ovalbumin gene per
haploid set of chromosomes, but the cellular mRNA is
long lived.
Translational and transcriptional control are sometimes
combined. For example,
insulin
(which regulates the syn-
thesis of a large number of substances) and
prolactin
(an-
other hormone) are required together for production of
casein (milk protein) in mammary tissue. Both hormones
are needed to initiate transcription but prolactin in addi-
tion, increases the lifetime of casein mRNA.
The synthesis of some proteins is regulated by direct
action of the protein on the mRNA. For instance, the con-
centration of one type of immunoglobulin is kept con-
stant by self-inhibition of translation. This protein, like
all immunoglobulins, consists of two H chains and two L
chains. The tetramer binds specifically to H-chain mRNA
and thereby inhibits initiation of translation.
Regulation of Protein Activity
Many enzymes contain several subunits and are regulated
by a process known as
allostery.
A common arrangement
in enzymes is that the binding sites for the molecule that
is acted on (the substrate) and the inhibitor (which may
be the product) are located on different subunits. If bind-
ing of the inhibitor prevents binding of the substrate, the
information from a site on one subunit must somehow be
transmitted to the other subunit. This can be accomplished
by the following subunit interactions. Binding of the in-
hibitor molecule alters the shape of the subunit to which it
is bound, resulting in changes in the reactive sites on other
subunits. If the subunits remain in contact, all subunits ad-
joining the first will undergo a conformational change at
their respective subunit interaction sites, altering, in turn,
the substrate binding site of other subunits. Proteins ca-
pable of undergoing such conformational interactions are
called
allosteric proteins
(Chapter 7).
In mammalian cells,
cAMP
is called a second messen-
ger, because it regulates the activities of many proteins.
Furthermore, certain hormones and cAMP work in con-
cert to regulate enzymatic activities. Many hormones reg-
ulate metabolic processes, such as glucose metabolism
and calcium utilization, through binding to specific recep-
tors in the cell membranes of target cells. However, many
