Algae, Tree, Herbs, Bush, Shrub, Grasses, Vines, Fern, Moss, Spermatophyta, Bryophyta, Fern Ally, Flower, Photosynthesis, Eukaryote, Prokaryote, carbohydrate, vitamins, amino acids, botany, lipids, proteins, cell, cell wall, biotechnology, metabolities, enzymes, agriculture, horticulture, agronomy, bryology, plaleobotany, phytochemistry, enthnobotany, anatomy, ecology, plant breeding, ecology, genetics, chlorophyll, chloroplast, gymnosperms, sporophytes, spores, seed, pollination, pollen, agriculture, horticulture, taxanomy, fungi, molecular biology, biochemistry, bioinfomatics, microbiology, fertilizers, insecticides, pesticides, herbicides, plant growth regulators, medicinal plants, herbal medicines, chemistry, cytogenetics, bryology, ethnobotany, plant pathology, methodolgy, research institutes, scientific journals, companies, farmer, scientists, plant nutrition
Select Language:
 
 
 
 
Main Menu
Please click the main subject to get the list of sub-categories
 
Services offered
 
 
 
 
  Section: Genetics » Mutations » Biochemical Level (Biochemical and Microbial Genetics)
 
 
Please share with your friends:  
 
 

Biochemical mutations and biosynthetic pathways

 
     
 
Content
Mutations : 2.  Biochemical Level (Biochemical and Microbial Genetics)
Inborn errors of metabolism in man
Eye transplantation in Drosophila
Biochemical mutations in Neurospora
Mutations in E. coli for resistance against phages or antibiotics
Cell counting in suspension
Calculation of mutation rates and frequencies
Biochemical mutations and biosynthetic pathways
Gene sequences and enzyme sequences in biosynthetic pathways
As shown earlier in case of synthesis of eye pigment in Drosophila, a biosynthetic pathway can be worked out using biochemical mutations, since mutation in a particular gene leads to block in a particular step of biosynthetic pathway. The step which has been blocked can be known either by accumulation of an intermediate product or by recovery of trait due to supply of another intermediate product. This will be illustrated by two additional examples.

Arginine synthesis in Neurospora
In Neurospora, following three kinds of mutations for arginine synthesis were isolated.
  1. Mutations, which grow only when arginine is supplied and do not grow with the help of either ornithine or citrulline alone.
  2. Mutations, which grow only when either citrulline or arginine is supplied, but do not grow when only ornithine is supplied.
  3. Mutations which can grow only, when either ornithine or citrulline or arginine is supplied.

These mutants indicate that, in the first case : (i) mutants are incapable of utilizing ornithine or citrulline; in the second case, (ii) mutants are incapable of utilizing ornithine; but in the last case, (iii) only the precursor cannot be utilized. The biosynthetic pathway, suggested on the basis of these mutation studies, is shown in Figure 22.8.
 
Different steps in the synthesis of arginine and three different mutations blocking three steps.
Fig. 22.8. Different steps in the synthesis of arginine and three different mutations blocking three steps.


Synthesis of tryptophan in bacteria and Neurospora
Salmonella typhimurium. In Salmonella typhimurium, four kinds of mutants for tryptophan synthesis could be isolated. These mutations are listed in Table 22.2, and include the following : (i) trp-8 does not have the ability to synthesize anthranilic acid but the growth can be restored by addition of either of the following compounds—anthranilic acid, indole glycerol phosphate, indole or tryptophan. (ii) trp-2 and trp-4 can not grow on either the minimal medium or on the supplemented medium with anthranilic acid, but can grow on addition of either indole glycerol phosphate, or indole or tryptophan. (iii) trp-3 can grow only on a supplemented medium with either indole or tryptophan, but not on any other medium, (iv) trp-1, 6, 7, 9, 10, 11, etc. can grow only on a supplemented medium with tryptophan and on no other medium. These different mutants indicate interruption of biochemical pathway at different steps.

genteic botany Biocyclopedia.com

Escherichia coli. In Escherichia coli also five different kinds of mutations called trp A, trp B, trp C, trp D and trp E were identified. These mutations are similar to those described for Salmonella, and are shown in Figure 22.9 and are summarized in Table 22.2. The biosynthetic pathway shown in Figure 22.9 was traced out using the mutants described above for Salmonella typhimurium and Escherichia coli.

Different steps in the synthesis of amino acid tryptophan, showing the enzymes and genes for each step.
Fig. 22.9. Different steps in the synthesis of amino acid tryptophan, showing the enzymes and genes for each step.

Tryptophan synthetase in Neurospora and bacteria. It has been shown that the last two steps of tryptophan metabolism in Neurospora as well as in bacteria are controlled by the same enzyme tryptophan synthetase, so that even mutants lacking the ability to utilize indole do not have tryptophan synthetase enzyme. In Neurospora, this enzyme consists of only one component. But in E. coli and Salmonella, it has two components : (i) 'A' component affecting penultimate step in Figure 22.9, so that trp A mutants lacking A component can be supplemented by both indole and tryptophan and (ii) 'B' component, which affects last step in Figure 22.9, so that trpB mutants lacking B component must be supplemented directly with tryptophan. It has also been shown that in mutants for this enzyme, protein is synthesized, but it is unable to perform enzymatic function.

The presence of two components (A and B) of tryptophan synthetase enzyme, synthesized under the control of two different genes suggests that a particular enzyme may consist of more than one polypeptides, each synthesized under the control of a different functional unit called cistron (see Fine Structure of Gene-at the Genetic Level (A New Concept of Allelomorphism)). Therefore, it was suggested that the concept of one gene-one enzyme proposed initially by Beadle and Tatum should be modified into one cistron-one polypeptide hypothesis.

 
     
 
 
     




     
 
Copyrights 2012 © Biocyclopedia.com | Disclaimer