WHAT IS A PROTOTROPH AND WHAT ARE ITS APPLICATIONS? - BIOLOGY - 2025

The prototrophs are organisms or cells that are capable of producing the amino acids that require for their life processes. This term is generally used in relation to a particular substance. It is opposite of the term auxotroph.

This last term is used to define a microorganism that is capable of growing and multiplying in a culture medium only if a specific nutrient has been added to it. In the case of the prototroph, it can thrive without such a substance because it is capable of producing it itself.

Simple comparison between auxotroph and prototroph. Taken and edited from: Akardoust, from Wikimedia Commons.

An organism or a strain, for example, unable to grow in the absence of lysine, would be called auxotrophic lysine. The prototrophic lysine strain, for its part, will grow and can reproduce independently of the presence or absence of lysine in the culture medium.

Basically, an auxotrophic strain has lost a functional metabolic pathway that allowed it to synthesize a fundamental substance, essential for its vital processes.

This deficiency is generally due to a mutation. The mutation generates a null allele that does not have the biological capacity to produce a substance present in the prototroph.

Applications

Biochemistry

Auxotrophic genetic markers are often used in molecular genetics. Each gene contains the information that codes for a protein. This was demonstrated by researchers George Beadle and Edward Tatum, in their work that earned them the Nobel Prize.

This specificity of the genes allows the mapping of biosynthetic or biochemical pathways. A mutation of a gene leads to a mutation of a protein. In this way, it can be determined in the auxotrophic strains of the bacteria under study which enzymes are dysfunctional due to mutations.

Another method to determine biosynthetic pathways is the use of auxotrophic strains of specific amino acids. In these cases, the strains' need for such amino acids is exploited to add unnatural amino acid analogs of the proteins in the culture media.

For example, substitution of phenylalanine for para-azido phenylalanine in cultures of Escherichia coli strains auxotrophic for phenylalanine.

Auxotrophic markers

Mutations within genes encoding enzymes that participate in pathways for the biosynthesis of metabolic building molecules are used as markers in the vast majority of genetic experiments with yeast.

The nutritional deficiency caused by the mutation (auxotrophy) can be compensated by supplying the required nutrient in the growth medium.

However, such compensation is not necessarily quantitative because mutations influence various physiological parameters and can act synergistically.

Because of this, studies have been conducted to obtain prototrophic strains with a view to eliminating auxotrophic markers and reducing bias in physiological and metabolic studies.

The Ames test

The Ames test, also called the Salmonella mutagenesis test, was developed by Bruce N. Ames in the 1970s to determine if a chemical is a mutagen.

It is based on the principle of reverse mutation or posterior mutation. It uses multiple strains of Salmonella typhimurium auxotrophic to histidine.

The power of a chemical to cause mutation is measured by applying it to bacteria on a plate containing histidine. The bacteria are subsequently moved to a new histidine-poor plaque.

If the substance is not mutagenic, the bacteria would not show growth on the new plaque. In another case, the histidine auxotrophic bacteria will again mutate to prototrophic histidine strains.

Culture of the prototrophic bacterium Salmonella typhimurium. Taken and edited from: Sun14916, from Wikimedia Commons

Comparison of the proportion of bacterial growth in plates with and without treatment makes it possible to quantify the mutagenic power of the compound on bacteria.

This possible mutagenic effect in bacteria indicates the possibility of causing the same effects in other organisms, including humans.

It is believed that a compound that is capable of causing a mutation in bacterial DNA may also be capable of producing mutations that can cause cancer.

Other applications to the Ames test

Development of new strains

The Ames test has been applied to obtain new bacterial strains. For example, nitroreductase-deficient strains have been developed.

These strains are used to study xenobiotic metabolism and DNA repair systems. They have also been useful to assess the metabolic mechanisms of nitrogroups to produce active mutagens, as well as the nitration mechanisms of genotoxic compounds.

Antimutagenesis

The Ames test has also been used as a tool to study and classify natural antimutagens. Antimutagens are compounds that can reduce mutagenic lesions in DNA, mainly by improving their repair systems.

In this way, such compounds avoid the initial steps of cancer development. Since the early 1980s (20th century), Ames et al. Have conducted studies to evaluate genotoxin reductions and cancer risks through a diet rich in antimutagens.

They observed that populations that had diets with high levels of antimutagens had lower risks of developing gastroenteric cancer.

The Ames test has been used extensively to study various plant extracts known to reduce mutagenicity. These studies have also shown that plant components are not always safe. Many edible plants have been shown to have genotoxic effects.

The Ames test has also been shown to be useful in detecting the toxic or antimutagenic effects of natural compounds that are frequently used in alternative medicine.

Genotoxic metabolism studies

One of the weaknesses of the Ames test was the lack of metabolic activation of genotoxic compounds. However, this problem has been solved by the addition of CYP-induced liver homogenates prepared from rodents.

CYP is a hemoprotein associated with the metabolism of various substances. This modification added new capabilities to the Ames test. For example, various inducers of CYPs have been evaluated, which showed that these enzymes are induced by different types of compounds.

Evaluation of mutagens in biological fluids

These tests use urine, plasma and serum samples. They can be useful to evaluate the formation of N-nitroso compounds in vivo from amino drugs.

They can also be useful in epidemiological studies of human populations exposed to occupational mutagens, smoking habits, and exposure to environmental pollutants.

These tests have shown, for example, that workers exposed to waste products have higher levels of urinary mutagens than those who worked in water treatment facilities.

It has also served to demonstrate that the use of gloves reduces the concentrations of mutagens in foundry workers exposed to polycyclic aromatic compounds.

Urinary mutagens studies are also a valuable tool for antimutagenic evaluation, since, for example, this test has shown that the administration of vitamin C inhibits the formation of N-nitroso compounds.

It also served to demonstrate that consuming green tea for a month reduces the concentration of urinary mutagens.

References

1. BN Ames, J. McCann, E. Yamasaki (1975). Methods for detecting carcinogens and mutagens with the salmonella / mammalian-microsome mutagenicity test. Mutation Research / Environmental Mutagenesis and Related Subjects.
2. B. Arriaga-Alba, R. Montero-Montoya, JJ Espinosa (2012). The Ames Test in Twenty-first Century. Research & Reviews: A Journal of Toxicology.
3. Auxotrophy. On Wikipedia. Recovered from
4. S. Benner (2001). Encyclopedia of Genetics. Academic Press.
5. F. Fröhlich, R. Christiano, TC Walther (2013). Native SILAC: Metabolic labeling of proteins in prototroph microorganisms based on lysine synthesis regulation. Molecular & Cellular Proteomics.
6. M. Mülleder, F. Capuano, P. Pir, S. Christen, U. Sauer, SG Oliver, M. Ralser (2012). A prototrophic deletion mutant collection for yeast metabolomics and systems biology. Nature Biotechnology.