The Questions of Developmental Biology

Control of Gene Expression at the Level of Translation
Once the RNA has reached the cytoplasm, there is still no guarantee that it will be
translated. The control of gene expression at the level of translation can occur by many means;
some of the most important of these are described below.
Differential mRNA longevity
The longer an mRNA persists, the more protein can be translated from it. If a message
with a relatively short half-life were selectively stabilized in certain cells at certain times, it would
make large amounts of its particular protein only at those times and places. The stability of a
message is often dependent upon the length of its poly(A) tail. This, in turn, appears to depend
upon sequences in the 3´ untranslated region. Certain 3´ UTR sequences allow longer poly(A)
tails than others. If these regions are experimentally traded, the half-lives of the resulting mRNAs
will be altered: usually long-lived messages will decay rapidly, while normally short-lived
mRNAs will remain around longer
(Shaw and Kamen 1986; Wilson and
Treisman 1988; Decker and Parker
1994). In some instances, messenger
RNAs can be selectively stabilized at
specific times in specific cells.
The mRNA for casein, the major
protein of milk, has a half-life of 1.1
hours in rat mammary gland tissue.
However, during periods of lactation, the
presence of the hormone prolactin
increases this half-life to 28.5 hours
(Figure 5.31; Guyette et al. 1979).
Selective inhibition of mRNA translation
Some of the most remarkable cases of translational regulation of gene expression occur in
the oocyte. The oocyte often makes and stores mRNAs that will be used only after fertilization
occurs. These messages stay in a dormant state until they are actived by ionic signals (to be
discussed in Chapter 7) that spread through the egg during ovulation or sperm binding. Table 5.3
gives a partial list of some of the mRNAs that are stored in the oocyte cytoplasm. Some of these
stored mRNAs are for proteins that will be needed during cleavage, when the embryo makes
enormous amounts of chromatin, cell membranes, and cytoskeletal components. Some of them
encode cyclin proteins that regulate the timing of early cell division (Rosenthal et al. 1980;
Standart et al. 1986). Indeed, in many species (including sea urchins and Drosophila),
maintenance of the normal rate and pattern of early cell divisions does not require a nucleus;
rather, it requires continued protein synthesis from stored maternal mRNAs (Wagenaar and
Mazia 1978; Edgar et al. 1989). Other stored messages encode proteins that determine the fates of
cells. These include the bicoid and nanos messages that provide positional information in the
Drosophila embryo, as we saw in Chapter 3, and the glp-1 mRNA of the nematode C. elegans.