Download GENE REGULATION IN HIGHER ORGANSIMS Although eukaryotes

GENE REGULATION IN HIGHER ORGANSIMS
Although eukaryotes do not have operons and are much more difficult to
study experimentally, examples of gene regulation and knowledge of the
molecular mechanisms involved are coming to light. Although eukaryotic
regulation is relatively more complex, many of the principles from the
lac-operon can be applied. For example, a number of promoters that
allow tissue specific expression thorough interaction with trans-acting
factors have been defined. One quite useful application in plants has
been the ability to place structural genes under the control of seedspecific promoters so that the protein can be made and stored in seeds.
The local company Prodigene www.prodigene.com is a pioneer in this
research, and is now selling corn that contains a valuable protein called
avidin that is widely used in basic biology research. A bushel of corn can
provide as much avidin as would normally be isolated from a boxcar full
of eggs, the usual source of this protein. Even better, the corn can be
stored without spoiling and the avidin can still be isolated in active form!
Lets look at a few examples from humans where gene regulation can be
seen to occur, even if all the molecular details are not available.
One interesting example involves the same enzyme that we called β galactosidase in bacteria. In humans, the enzyme is simply called lactase.
Since human milk is 7% lactose, it is fortunate that almost all babies are
born with lactase.
On aging however, many adults lose the ability to
digest lactose; they become lactose intolerant. If these individuals
consume lactose, they tend to develop gas, cramps and even diarrhea.
the ability of adults to digest lactose is culture-dependent. In general, in
dairy-oriented cultures adults maintain lactase, whereas in non-dairy
cultures they do not. For example, among Finns and Swedes 96 % of the
adults are lactose tolerant, while among Eskimos and Asians, 88-98 % of
adults are intolerant. Adults in seemingly dairy-oriented cultures where
the lactose is pre-digested by microbes to make cheese or yogurt also tend
to be intolerant. It is not clear if synthesis of the enzyme is shut down as
a consequence on non-exposure to milk, or simply as a function of aging.
It does not appear to be easy to turn the lactase gene back on once it has
been shut off.
A clear case of developmental regulation can be seen in our genes for
hemoglobin; the molecule in red blood cells that function to transport
oxygen. All hemoglobins are made of four polypeptide chains, which
almost always occur in two pairs. The 4 chains of amino acids - globinssurround the iron-containing heme group which binds to O 2 .
Adult humans typically have H b - A which has two alpha (α ) and 2 beta (β )
globin chains. Tha alpha and beta chains of amino acids come from
different genes. There are actually 2 adjacent alpha genes on
chromosome 16 in man and one beta gene on chromosome 11. We also
make a small amount of alpha2/delta2 hemoglobin as adults; the delta
globin gene is beside the beta globin gene. These genes are turned on in
bone marrow only, which is where all of our blood cells originate.
During most of the gestation period, a fetus has fetal hemoglobin or H b - F.
Hb-F is a combination of alpha2/gamma2 gene products. There are two
gamma genes just before the beta gene on chromosome 11; they differ by
just one nucleotide so that one has the amino acid glu where the other
has asp. The gamma genes are turned on starting at about 6 weeks after
conception and begin to turn off before birth. During this time, the active
globin genes are found in the liver.
Several embryonic forms of hemoglobin (Hb-E) are made during the first
6 weeks following conception. Active genes are transcribed in the yolk
sac during very early development. Along with alpha genes, there are
zeta and epsilon genes expressed during this developmental period.
The various forms of Hb have differing affinities for O2 ; the embryonic
forms function in a relatively low O2 environment and the fetal globins
must be able to take O2 from the mothers blood, as compared to adult Hb
where the O 2 is absorbed in the lungs.
There are a number of genetic defects where not enough hemoglobin is
made. These can result from defects in the gene(s) or from defective
regulation. The common name for these diseases is thalassemia. If no
alpha globin is made the disease is alpha0 -thalassemia and if some but less
than the normal amount is made, the disease is called alpha + -thalassemia.
Beta0 and Beta+ types of thalassemia also occur.
In all cases, individuals who are heterozygous for one normal gene and
one defective allele may have a minor degree of anemia, but are
essentially normal. Heterozygotes are often said to have thalassemia
minor, while homozygous defective individuals have thalassemia major.
If n o alpha globin is made, the condition is known as hydrops fetalis and
is usually lethal within 24 hours after birth.
When no beta globin is made, the condition is called Cooley's anemia or
sometimes Mediterranean anemia reflecting the fact that it is relatively
common in that area of the world. In fact, it is the cause of many
thousands of childhood deaths per year around the globe. Patients have
all the problems associated with severe anemia. Their system tries to get
along with fetal hemoglobin but these genes naturally turn off. Very
limited O2 transport can be handled by alpha4 globin, but soon the only
solution is blood transfusions every 2-3 weeks. After some years of
transfusions, the build-up of iron damages the liver heart and kidneys, so
that patients seldom survive into their thirties. (The OMIM entry for β 0 thalassemia is: <http://www.ncbi.nlm.nih.gov:80/entrez/dispomim.cgi?id=141900>
In cases where an HLA match can be found, a bone marrow transplant can
effectively cure the symptoms.
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Many different defects in the promoter or β -globin structural gene have
been described that lead to deficiency of globin production, including
deletions, new stop codons and changed intron splice signals.
An interesting observation was made in patients who have a condition
called "Hereditary Persistence of Fetal Hemoglobin (HPFH). These
individuals get along relatively well with only fetal hemoglobin being
made, even as adults. In some cases, the whole beta gene and the
adjacent delta gene have been shown to be entirely missing in these
individuals. This suggests that there may be some regulatory factor
within or between these genes that normally functions to turn off the fetal
gamma-type globins. HPFH also has spurred efforts to try to turn on the
fetal Hb genes in adults with thalassemia major. One compound that can
do so, at least for short a time is 5-azacytidine, a base analog that is rather
toxic so can not be used without very careful monitoring. The rationale
for testing 5-azacytidine came from its known ability to cause
demethylation of DNA. It turns out that many of the Cs in the promoters
of genes that are inactive are methylated (5-m C) but that when the same
gene is active, the same Cs are not methylated. (This includes our globin
genes). At this point it is still very difficult to determine whether
methylation is a method for silencing genes, or a consequence of the gene
already being inactive. In any case removing the methyl from 5m Cs leads
to increased transcription. Again however, just as no method for
selectively methylating specific promoters has been identified, 5-azaC is
not specific in removing methyl groups. Thus, other genes that should
remain inactive may be turned on when attempting to treat thalassemia.
The treatment is used only in patients who can no longer have
transfusions.
In general the models for eukaryotic gene regulation are similar to the
lac-operon, except that multiple trans-acting factors may attach to the
same promoter; some of them stimulate transcription while other tend to
decrease transcription. Thus transcription of a tissue specific or
developmentally expressed gene may depend on a delicate balance of
multiple interacting factors.
We know that hormones are key factors in gene regulation in plants and
animals. There are two types of hormones -peptides and steroids. As the
name implies, peptide hormones are chains of amino acids and include
molecules such as insulin, nerve growth factor, human growth hormone
and many others. Peptide hormones do not enter the target cell but
instead interact with receptors found on the cell surface. Once they bind,
a reaction occurs on the inner side of the membrane to release another
compound that is referred to as a "second messenger". In many cases the
2 nd messenger is cAMP, the same compound that signaled well-off
conditions in bacteria to prevent induction of the lac operon if both
glucoses and lactose are present. The second messenger can then interact
with other proteins to cause a phosphate to be added or removed from
another protein, setting up a cascade of steps eventually leading to active
transcription factors that interact directly with DNA. Much research is
underway to understand these signal transduction pathways,
Steroid hormones such as estrogens and testosterone can directly enter
the cell. Inside, they still tend to interact with specific receptors in order
to regulate transcription.
Since hormones tend to stimulate mRNA synthesis, the target sequence in
the promoter that is recognized by the transcription factor (hormone &
receptor or second messenger molecule) is generally referred to as an
enhancer element. Many genes also have been found to have interacting
"silencers" where binding of a transcription factor can prevent
transcription.
To be thorough, it should be noted that gene expression can also be
regulated by regulating translation or even after translation by adding
glucoses, phosphates, etc. to specific proteins. Many proteins such as
insulin are first made as larger molecules that require specific cuts by
other proteases to become active. Likewise, some proteins have a much
longer "lifetime" than others, so the rate of turnover can also be
important in regulating gene expression.