selectable markers
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A selectable marker is a gene introduced into a cell, especially a bacterium or to cells in culture, that confers a trait suitable for
artificial selection Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
. They are a type of reporter gene used in laboratory
microbiology Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...
, molecular biology, and
genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including t ...
to indicate the success of a transfection or other procedure meant to introduce foreign DNA into a cell. Selectable markers are often
antibiotic resistance Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...
genes (''An antibiotic resistance marker is a gene that produces a protein that provides cells expressing this protein with resistance to an antibiotic.''). Bacteria that have been subjected to a procedure to introduce foreign DNA are grown on a medium containing an antibiotic, and those bacterial colonies that can grow have successfully taken up and expressed the introduced genetic material. Normally the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for ''E. coli''.


Modus operandi

The non-recombinants are separated from recombinants; i.e., a r-DNA is introduced in bacteria, some bacteria are successfully transformed some remain non-transformed. When grown on medium containing ampicillin bacteria die due to lack of ampicillin resistance. The position is later noted on nitrocellulose paper and separated out to move them to nutrient medium for mass production of required product. An alternative to a selectable marker is a screenable marker which can also be denoted as a reporter gene, which allows the researcher to distinguish between wanted and unwanted cells, e.g. between blue and white colonies. These wanted or unwanted cells are simply un-transformed cells that were unable to take up the gene during the experiment.


Positive and Negative

For molecular biology research different types of markers may be used based on the selection sought. These include: *Positive or selection markers are selectable markers that confer selective advantage to the host organism. An example would be antibiotic resistance, which allows the host organism to survive antibiotic selection. *Negative or counterselectable markers are selectable markers that eliminate or inhibit growth of the host organism upon selection. An example would be thymidine kinase, which makes the host sensitive to ganciclovir selection. *Positive and negative selectable markers can serve as both a positive and a negative marker by conferring an advantage to the host under one condition, but inhibits growth under a different condition. An example would be an enzyme that can complement an auxotrophy (positive selection) and be able to convert a chemical to a toxic compound (negative selection).


Common examples

Examples of selectable markers include: * Beta-lactamase which confers ampicillin resistance to bacterial hosts. *Neo gene from Tn5, which confers resistance to kanamycin in bacteria and
geneticin G418 (Geneticin) is an aminoglycoside antibiotic similar in structure to gentamicin B1. It is produced by '' Micromonospora rhodorangea''. G418 blocks polypeptide synthesis by inhibiting the elongation step in both prokaryotic and eukaryotic ce ...
in eukaryotic cells *Mutant FabI gene (mFabI) from ''E. coli'' genome, which confers triclosan resistance to the host. *'' URA3'', an orotidine-5' phosphate decarboxylase from yeast is a positive and negative selectable marker. It is required for uracil biosynthesis and can complement ''ura3'' mutants that are auxotrophic for uracil (positive selection). The enzyme URA3 also converts 5-fluoroorotic acid (5FOA) into the toxic compound
5-fluorouracil Fluorouracil (5-FU), sold under the brand name Adrucil among others, is a cytotoxic chemotherapy medication used to treat cancer. By intravenous injection it is used for treatment of colorectal cancer, oesophageal cancer, stomach cancer, pancrea ...
, so any cells carrying the ''URA3'' gene will be killed in the presence of 5FOA (negative selection).


Future developments

In the future alternative marker technologies will need to be used more often to, at the least, assuage concerns about their persistence into the final product. It is also possible that markers will be replaced entirely by future techniques which use removable markers, and others which do not use markers at all, instead relying on co-transformation, homologous recombination, and recombinase-mediated excision.


References

{{Use dmy dates, date=August 2017 Genetics techniques Molecular biology