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Transgenics

TRANSGENICS

Transgenic organisms contain an isolated or artificially synthesized foreign DNA or gene or extra copies of an endogenous gene (transgene) in their cell either as integrated into its genome or into a vector. Thus, they are genetically modified organisms. Foreign DNA has been successfully introduced in many microorganisms, plants and animals to utilize them for the welfare of human beings for producing desired phenotype or protein or to modify the existing biosynthetic pathway to get new product or to inhibit the expression of a gene.

1. Transgenic Microorganisms

Some important transgenic microorganisms are:

i.            E. coli containing human insulin gene has been used to produce insulin for the treatment of diabetes. Earlier, insulin was extracted from pigs and cattle which had some side effects due to additional contaminating compounds, whereas, the recombinant insulin, produced by recombinant E. coli introduced into the market in 1982, has no such problems as it is exactly similar to human insulin.

ii.            Somatostatin, a proteinaceous growth hormone, was introduced into the market in 1985. It is extremely difficult to isolate from the animals (about 0.005 g hormone is isolated from 5 lakh sheep brains). After cloning of the human growth hormone gene in E. coli, the yield has been increased many times (0.005 g/L bacterial fermentation medium). It is used for the treatment of hypopituitary dwarfism, which occurs due to growth hormone deficiency (with a frequency of about 1 child in 5000). The hormone improves the muscle growth and is exploited by some athletes.

iii.            Moreover, interferons, blood clotting factor, etc. are also produced by genetically engineered microbial cells or cultured mammalian cells. Interferons were discovered by two British researchers, Issacs and Lindenmann in 1957. They have been named interferons as they interfere with the virus infection by inducing degradation of viral RNA in infected cells by the enzyme ribonuclease. Human interferons are glycoproteins protein + sugar) and are produced by virus-infected cells to protect healthy cells. They control a large number of viral diseases, e.g., common cold, cancer, etc. They have the potential application in cancer therapy as they attack cancer cells by inhibiting their growth (division) and also check the growth of viruses causing cancer virus, AIDS, hepatitis-B, etc. The protection conferred by them is non-specific (interferons induced by one virus can provide protection against other viruses also). They have not become the antibiotics of the viral infections because minute amounts of interferons are produced in the cells and, moreover, their extraction from other cellular proteins is complicated. Conventionally, they were obtained from the human blood. But currently, they are produced from cultured mouse fibroblasts or human leucocytes infected with Sendai virus and genetically engineered bacteria (E. coli) or yeast with inserted gene for human fibroblast interferon, which are not only more pure but much cheaper also (about VI50 cost of the conventional method) than the conventionally obtained interferons.

iv.            Recombinant vaccines are produced through recombinant DNA technology by integrating the gene for the immunogenic proteins in E coli, yeast, cultured animal cells or transgenic plants. Here, DNA segment necessary for the production of either:

a)      whole protein (e.g., coat protein gene for hepatitis-B has been cloned in yeast) or

b)      a part of a protein (e.g., amino acid 114-160 of coat protein for foot and mouth disease virus) necessary for immunogenicity, is cloned.

But, these protein vaccines (second revolution in vaccines) are very costly and need to be stored at low temperatures. (First revolution in vaccine was the production of conventional vaccines. Third revolution in vaccine is the use of DNA in the form of vaccine that  after injection into the blood expresses to produce necessary immunogenic protein.)

v.            Saccharomyces cerevisiae containing dextrin-utilizin gene from Saccharomyces diasticus by recombinant DNA technology (normally the yeast cannot utilize dextrins) increases the amount of fermentable sugars and, thus, greater amount of alcohol after the fermentation is produced. (Dextrins are small chains of glucose.)

vi.            Rennet, used in cheese industry, is traditionally obtained from the pancreas of calves but now can also be obtained from the genetically engineered microbes (e.g., Escherichia coli) containing the calf chymosin (rennin) gene.

2. Transgenic Plants

Man is dependent on agriculture for food and other requirements and many countries are still not self-sufficient in food production. Biotechnology is creating a revolution in this field by providing higher quality and lower cost products for continuously increasing human population. Since early times, man is trying to improve the quality and productivity of agriculturally important plants by selection of better varieties and employing the traditional breeding programmes involving sexual crosses, which are slow and difficult processes. Extensive research is also going on with forest trees, as forest resources are continually depleting because of indiscriminate deforestation. Trees have long generation times and desired traits can be achieved by the faster tissue culture and genetic engineering technologies.

The genetic engineering technology overcomes the limits of interspecific crosses and permits the introduction of foreign genes from distant species that is not possible following conventional breeding programmes. It is now possible to manipulate the genome of almost any plant species including agricultural crop plants by introducing advantageous new traits (by incorporating isolated gene from an organism or chemically synthesized gene) that either products a new protein/other product or prevents the expression of an existing native gene. For this, many techniques can be employed.

One such important method is the use of Agrobacterium tumefaciens that has been used extensively to transfer the desired foreign DNA by inserting it into the Ti plasmid and then integrating into the dicotyledonous host plant genome. After screening of the desired recombinant cells, they are regenerated into complete plantlets, e.g., Loblolly pine (used in forestry). Ti plasmids have also been used to insert ‘antisense genes’ to negate the functions of specific plant genes concerned with an undesirable trait. Such genes are produced by reversing the orientation of a gene in relation to its promoter (in DNA at promoter region, RNA polymerase binds to start the transcription giving rise to mRNA) so that a RNA, that is complementary to mRNA, is synthesized and after base pairing with the normal mRNA, double-stranded mRNA-antisense RNA is produced that is unable to be translated to give the protein.

This method of using Agrobacterium tumefaciens to manipulate the genome is unsuitable for

monocotyledonous plants, like wheat, rice, maize, etc. In monocotyledonous plants, gene transfer is mostly achieved with the help of a gene gun by bombarding foreign DNA-coated gold or tungsten particles (micro projectiles) into the host plant cells. The foreign DNA along with microprojectiles penetrates the cell wall and the DNA is delivered into the nucleus. The recombinant cells can be regenerated into the whole plants.

The widespread use of insecticides, fungicides, herbicides (to kill the plant weeds) and pesticides has the damaging effect on the environment and human health. Many crop plants have not only been improved with respect to the pest and disease control but also to increase the yield and quality of the product by genetic engineering technology:

        i.            Resistance to Insect Pests: Proteinacious toxin genes (cry genes) from Bacillus thruringiensis (Bt), that is toxic to larvae of certain insects, have been introduced into several crop plants so that they become resistant to that particular insects, e.g., cotton, tomato. In India also, some insect resistant cry genes containing cotton varieties are grown in the fields.

      ii.            Resistance to Microbial Diseases: Microbial diseases, particularly fungal and viral, cause huge losses and limit the agricultural productivity worldwide. Resistance to many viruses, by integrating the genes for viral coat proteins, have achieved in several crop plants, e.g., rice.

    iii.            Resistance to Herbicides: Herbicide-tolerant crop plants have been produced by genetic engineering the crop plant genomes that can reduce the application of herbicides, e.g., potato, rapeseed, etc. This method of weed control is more effective, less costly and more environment friendly.

    iv.            Delayed Ripening Fruits: Losses, during storage and transportation, of some crops are very high due to over-ripening and softening of fruits and vegetables. The ripening is due to some endogenous enzyme activities that can be genetically stopped or slowed down, e.g., in tomato, the enzymes polygalacturonase breaks down the cell wall leading to softening of fruit during the ripening. Flavr Savr tomato has been engineered in USA by blocking the expression of polygalactarunase enzyme by introducing the antisense gene for this particular enzyme. This tomato has improved flavour and shelf life and is now marketed in USA (Fig. 12.21). Such techniques arc now tried to be used in a wide variety of soft fruits. Other studies are considering the inhibition of ethylene synthesis for controlling the fruit ripening (ethylene is a plant hormone that induces fruit ripening).

      v.            Stress Resistance: Cold resistant tobacco plant has been produced by introducing the gene for glycerol-1-phosphate acyl transferase from the plant Arabidopsis.

Fig-12.21

Fig. 12.21 Introduction of antisense gene for polygalacturonase enzyme

for the development of Flavr Savr tomato.

 Despite their beneficial effects, it has been argued that transgenic plants may be harmful the environment as:

i.            There may be possibility of transfer of transgene (foreign gene) from transgenic crops to their wild relatives through pollens making them more persistent and damaging. To prevent this, transgenics should not be grown near the occurance of their wild relatives.

ii.            Transgenic crops may themselves become persistent weeds.

iii.            They may damage the environment in some unknown manner.

3.Transgenic Animals

Though transgenics have been produced in a variety of animals, most or the activities the experimental stage and have not commercially utilized yet. Some important transgenic animal

i.            In some animals, genes have been transferred and expressed into cultured cells to obtain important proteins, like human growth hormone from genetically modified mouse mammary tumour cells blood clotting factor VIII, etc.

ii.            Genetic modification of some animals may increase the milk (e.g., transgenic cow), meat (e.g., by introducing growth hormone gene in fish, goat, cow, pig, etc), wool (e.g., in sheep, etc.) production.

iii.            In some animals (e.g., transgenic rabbits, cows, goats, etc.), important genes have been transferred to get large amount of rare and expensive pharmaceutical or biologically important proteins in their milk, urine or blood. This technology is known as molecular farming. Generally, the transgenes are desirable to be expressed in mammary tissue so that the protein is secreted in their milk from which it can be easily isolated.