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2 Improvement of biochemical pathways in the body Nutrition studies have shown that some of the nutrients are necessary for growth and development and must be met by external supplies, such as lysine. The emergence of transgenic technology allows the animal to synthesize lysine itself, which is to change the animal's metabolic pathways. Wand et al. (1986) proposed that this method has two directions, one is to reconstruct some lost metabolic pathways; the other is to introduce metabolic pathways that have not yet been found in animals.
Dr. Bawden’s team reported in 1995 that they successfully expressed cytocysteine ​​biosynthetic genes in transgenic mice and sheep in an attempt to improve wool growth. Cysteine ​​is a limiting amino acid for wool synthesis. In 1990, Rogers et al. studied the mechanism of the increase of cystine conversion rate by the exogenous amino acid acetyltransferase and O-acetyl amino acid sulfhydrylase genes. Since cysteine ​​is degraded in the rumen of the sheep, the addition of cysteine ​​to feed does not increase its levels in sheep serum. If you can get a transgenic sheep that can self-synthesize cysteine, wool production will be greatly improved. For this purpose, the cystine-to-acetyl and O-acetyl amino acid sulfhydrylase genes and suitable regulatory elements in E. coli can be recombined and introduced into sheep and expressed therein. The gastric epithelial cells of this transgenic sheep use the hydrogen sulfide in the stomach to synthesize cysteine. Wand et al. (1990-1991) linked these genes to the metallothionein gene (MT) promoter and put the sequence of the GH gene on the 3' end. Using this construct, transgenic mice were obtained.
Similar to the successful transfer of the cysteine ​​synthesis pathway, Rees et al. (1990) proposed the idea that the pathway for the biosynthesis of threonine and lysine in microorganisms is introduced into mammals, so that mammals themselves Essential amino acids such as lysine and threonine can be synthesized. Through molecular biology methods, the essential amino acid content of livestock and poultry diets can be greatly reduced. Current microbiological studies have understood the gene coding for enzymes in the synthesis of threonine and lysine pathways in E. coli, and the use of gene transfer techniques has also demonstrated that it is possible to perform these pathways in cells. Ali et al. (1995) reported the success of the introduction of the endogenous glucanase E' gene in mice. This gene encodes an endogenous glucanase from Clostridium thermocellum, and in this transgenic model, the glycosylphosphatidyl inositol regulatory signal directs secretion of the enzyme in the small intestine. Although the amount of secretion is small, a new idea for the use of cellulose by animals has been proposed.
3 Changes in animal product composition and quality control The emergence of genetically modified animals has made the animal product composition changes and quality control has a broader perspective. Choosing the mammary gland as an organ for specific recombinant proteins allows for easy collection of products without damaging individual animals. The introduction of appropriate foreign genes to alter the composition of milk, and the use of milk-producing animals as a "bioreactor" is currently an important issue. The whey protein gene, sheep β-lactoglobulin gene, bovine asl casein gene, and β-casein gene have been constructed. Buhler et al. (1990) used the rabbit β-casein gene as a promoter and the human interleukin 2 gene as a target gene to produce a transgenic rabbit that secretes human interleukin 2 in milk (interleukin 2 is a clinical marker). In the treatment of cancer drugs). In 1985, the Institute of Animal Physiology and Genetics of Edinburgh first studied the successful expression of human coagulation factor IX gene in ewes milk. Zhang Kezhong et al. (1997) also reported on the construction of a breast tissue-specific expression vector for the construction of human coagulation factor IX gene and the success of secretory expression in the milk gland of dairy goats. The human coagulation factor IX protein isolated and produced from milk using this technique can be used to relieve the suffering of hemophiliacs. Zhang Jingwei et al. (1997) reported the expression of hepatitis B virus surface antigen in goat milk using bovine α-S1-casein gene and its expression constructs. These results indicate that our country's mammary gland bioreaction research has made gratifying progress.
4 Research on the mechanism of action of nutrients In recent years, research on the mechanism of action of nutrients using transgenic animals has been conducted. Catherine and Robert (1997) reviewed the research on the mechanism of action of key enzymes in the process of fat metabolism using transgenic animals. Through a series of tests, it was shown that NPY is an important substance in the energy balance mediation. Wolf (1995) reviewed retinol transport and metabolism in several knockout TTR mice. Transgenic mice lacking TTR had low levels of retinol and RBP in plasma and no significant decrease in RBP in urine. The fact that retinol and RBP are very low in plasma is not due to increased loss in the kidney, but reduced secretion from the liver. At the same time, the levels of RBP and retinol complexes in livers of transgenic mice lacking TTR were higher than normal mice. These results indicate that the synthesis of TTR in the liver favors the secretion of RBP and retinol into the plasma, that is, after TTR is secreted in the liver, it is combined with RBP and retinol complex to form a ternary complex, which is beneficial to RBP and The retinol complex is transported into the plasma.
1 Regulating growth of animal growth and carcass composition is a complex process. It is affected by interactions between various hormones, active factors, and cofactors. It is also affected by nutritional conditions and environmental factors, among which are proteins encoded by genes. Hormones are particularly important, such as growth hormone (GH), insulin-like growth factor-I (IGF-I) and so on. At present, GH has the most extensive and deepest research in controlling circulating peptide hormones and growth factor genes. GH is a 190 or 191 amino acid long peptide chain hormone secreted from the anterior pituitary of pigs, which accelerates protein synthesis and regulates the metabolism of carbohydrates and lipids. GH is currently used for injection in animal production. It also has certain effects, but the procedure is complex and cumbersome. One of the solutions is to use transgenic technology. Under normal circumstances, circulating GH levels are mainly regulated by the hypothalamus, and the heterologous promoter/enhancer is first used to reconstitute and assemble the fusion gene in vitro to avoid regulation of the hypothalamus. Through the new promoter/enhancer of the endogenous pathway, exogenous growth hormone can be specifically and indirectly expressed in vivo. By increasing the amount of synthetic GH, the purpose of promoting growth and reducing the ratio of feed to meat is achieved. (Yong Yongfu et al. (1988) bred pigs with MT/PGH (goat metallothionein promoter/porcine somatotropin gene) bred to obtain 20%-50% faster primary transgenic pigs than the control group; OMT/PGH (sheep metallothionein promoter/porcine somatotropin gene) pigs had a daily gain of 1000 g at the peak of growth, a feed to meat ratio of 2.5:1, and a lean percentage increase of about 15%. Wei Qingxin et al. (1995) introduced the OMT/PGH fusion gene constructed by Chen Yongfu into Hubei White Pig and obtained the transfer OMT/PGH pig. The results of the growth test were: crude protein 18%, lysine 0.9%, pulsed zinc 1600 mg/kg The daily gain of transgenic pigs was 11.8% higher than that of the litter control group, G1 (G0G0 or G0 non-transgenic pigs) increased by 14.2%, the feed conversion rate was increased by 10%, and the lean percentage was increased by 2 percentage point. Chen Qingxuan et al (1997) used plasmid pUC19 to construct an expression vector pSMTpGH using the promoter of the sheep MT-la gene as a promoter, and microinjection to obtain transgenic pigs, goldfish and mice. Under the same conditions of feeding, after the induction with ZnCl2 (the expression of the latter gene was started under the induction of zinc ion in the MT promoter), the growth rate of 30-day-old transgenic mice was increased by 50%, and the growth rate of 1-year-old transgenic goldfish was increased. Increased by 11.8%, growth rate of transgenic pigs increased by 14.8%, feed conversion rate increased by 10%, and lean meat percentage increased by 2%. Contrary to the above results, in some trials, some animals that had been transgenic for growth hormone did not show a growth-promoting effect. However, the most obvious change in transgenic pigs is the loss of fat, which reduces the thickness of backfat from 18 to 20 mm to 7 to 8 mm. In addition to increasing the growth rate, growth hormone also improves carcass quality, increases protein content, and reduces fat content. In addition, Hubbard et al. (1994) reported that transfection of a cDNA (complementary DNA) hepatic lipase gene expressing one or more mice in mice is a feasible approach to reduce fat storage. Weighart et al. (1990) showed that genetic modification can increase lean meat percentage and improve carcass quality.