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. They are a type of
reporter gene used in laboratory
microbiology,
molecular biology, and
genetic engineering to indicate the success of a
transfection or other procedure meant to introduce foreign
DNA into a cell. Selectable markers are often
antibiotic 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.[citation needed]
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.[1] 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.[2] An example would be
thymidine kinase, which makes the host sensitive to
ganciclovir selection.[citation needed]
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).[citation needed]
Neo gene from Tn5, which confers resistance to
kanamycin in bacteria and
geneticin in eukaryotic cells[3]
Mutant FabI gene (mFabI) from E. coli genome, which confers
triclosan resistance to the host.[4]
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, so any cells carrying the URA3 gene will be killed in the presence of 5FOA (negative selection).[5]
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.[6]