AUXOTROPH: ORIGIN, EXAMPLE AND APPLICATIONS - BIOLOGY - 2024

An auxotroph is a microorganism that is not capable of synthesizing a certain type of nutrient or organic component essential for the growth of said individual. Therefore, this strain can only proliferate if the nutrient is added to the culture medium. This nutritional requirement is the result of a mutation in the genetic material.

This definition generally applies to specific conditions. For example, we say that the organism is auxotrophic for valine, which indicates that the individual in question needs this amino acid to be applied in the culture medium, since it is not capable of producing it by itself.

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In this way, we can differentiate two phenotypes: "the mutant", which corresponds to the valine auxotroph - taking into account our previous hypothetical example, although it can be an auxotroph for any nutrient - and "the original" or wild, which can correctly synthesize the amino acid. The latter is called a prototroph.

Auxotrophy is caused by some specific mutation that leads to the loss of the ability to synthesize an element, such as an amino acid or other organic component.

In genetics, a mutation is a change or modification of the DNA sequence. Generally the mutation inactivates a key enzyme in a synthetic pathway.

How do auxotrophic organisms originate?

In general, microorganisms require a series of essential nutrients for their growth. Your minimum needs are always a carbon source, an energy source, and various ions.

Organisms that need extra nutrients to the basic ones are auxotrophs for this substance and originate from mutations in DNA.

Not all mutations that occur in the genetic material of a microorganism will affect its ability to grow against a particular nutrient.

A mutation may occur and it has no effect on the phenotype of the microorganism - these are known as silent mutations, since they do not change the sequence of the protein.

Thus, the mutation affects a very particular gene that codes for an essential protein of a metabolic pathway that synthesizes an essential substance for the body. The mutation generated must inactivate the gene or affect the protein.

It generally affects key enzymes. The mutation must produce a change in the sequence of an amino acid that significantly changes the structure of the protein and thus eliminates its functionality. It can also affect the active site of the enzyme.

Examples in

S. cerevisiae is a single-celled fungus popularly known as beer yeast. It is used for the manufacture of edible products for humans such as bread and beer.

Thanks to its usefulness and easy growth in the laboratory, it is one of the most widely used biological models, which is why it is known that specific mutations cause auxotrophy.

Auxotrophs for histidine

Histidine (abbreviated in the one-letter nomenclature as H and three-letter as His) is one of the 20 amino acids that make up proteins. The R group of this molecule is made up of a positively charged imidazole group.

Although in animals, including humans, it is an essential amino acid - that is, they cannot synthesize it and must incorporate it through diet - microorganisms have the ability to synthesize it.

The HIS3 gene in this yeast codes for the enzyme imidazole glycerol phosphate dehydrogenase, which participates in the pathway for the synthesis of the amino acid histidine.

Mutations in this gene (his3 ^-) result in histidine auxotrophy. Thus, these mutants are unable to proliferate in a medium lacking the nutrient.

Auxotrophs for tryptophan

Similarly, tryptophan is a hydrophobic amino acid that has an indole group as R group. Like the previous amino acid, it must be incorporated into the diet of animals, but microorganisms can synthesize it.

The TRP1 gene codes for the enzyme phosphoribosyl anthranilate isomerase, which is involved in the anabolic tryptophan pathway. When a change occurs in this gene, a trp1 mutation is obtained ^- which disables the body to synthesize the amino acid.

Auxotrophs for pyrimidines

Pyrimidines are organic compounds that are part of the genetic material of living organisms. Specifically, they are found in nitrogenous bases, forming part of thymine, cytosine and uracil.

In this fungus, the URA3 gene codes for the enzyme orotidine-5'-phosphate decarboxylase. This protein is responsible for catalyzing a step in the de novo synthesis of pyrimidines. Therefore, the mutations that affect this gene cause uridine or uracil auxotrophy.

Uridine is a compound that results from the union of the nitrogen base uracil with a ribose ring. Both structures are linked by a glycosidic bond.

Applications

Auxotrophy is a very useful characteristic in studies related to microbiology, for the selection of organisms in the laboratory.

This same principle can be applied to plants, where by genetic engineering an auxotrophic individual is created, either for methionine, biotin, auxin, etc.

Application in genetic engineering

Auxotrophic mutants are widely used in laboratories where genetic engineering protocols are performed. One of the goals of these molecular practices is the instruction of a plasmid constructed by the researcher in a prokaryotic system. This procedure is known as “auxotrophy complementation”.

A plasmid is a circular DNA molecule, typical of bacteria, that replicates independently. Plasmids can contain useful information that is used by the bacteria, for example resistance to some antibiotic or a gene that allows it to synthesize a nutrient of interest.

Researchers who want to introduce a plasmid into a bacterium can use an auxotrophic strain for a specific nutrient. The genetic information necessary for the synthesis of the nutrient is encoded in the plasmid.

In this way, a minimal medium (which does not contain the nutrient that the mutant strain cannot synthesize) is prepared and the bacteria are seeded with the plasmid.

Only bacteria that incorporated this portion of plasmid DNA will be able to grow in the medium, while bacteria that failed to capture the plasmid will die from lack of the nutrient.

References

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5. Molina, JLM (2018). 90 solved problems of Genetic Engineering. Miguel Hernández University.
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