In biology, a gene cassette is a type of

mobile genetic element Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...

that contains a gene and a recombination site. Each cassette usually contains a single gene and tends to be very small; on the order of 500–1000

base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both D ...

s. They may exist incorporated into an integron or freely as circular DNA. Gene cassettes can move around within an organism's genome or be transferred to another organism in the environment via

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...

. These cassettes often carry

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. ...

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s. An example would be the '' kanMX'' cassette which confers kanamycin (an

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...

) resistance upon

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

Integrons

Integrons are genetic structures in

which express and are capable of acquiring and exchanging gene cassettes. The integron consists of a promoter, an attachment site, and an

integrase Retroviral integrase (IN) is an enzyme produced by a retrovirus (such as HIV) that integrates—forms covalent links between—its genetic information into that of the host cell it infects. Retroviral INs are not to be confused with phage in ...

gene that encodes a site-specific

recombinase Recombinases are genetic recombination enzymes. Site specific recombinases DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria ...

There are three classes of integrons described. The mobile units that insert into integrons are gene cassettes. For cassettes that carry a single gene without a promoter, the entire series of cassettes is transcribed from an adjacent promoter within the integron. The gene cassettes are speculated to be inserted and excised via a circular intermediate. This would involve recombination between short sequences found at their termini and known as 59 base elements (59-be)—which may not be 59 bases long. The 59-be are a diverse family of sequences that function as recognition sites for the site-specific integrase (enzyme responsible for integrating the gene cassette into an integron) that occur downstream from the gene coding sequence.

Diversity and prevalence

The ability of genetic elements like gene cassettes to excise and insert into genomes results in highly similar gene regions appearing in distantly related organisms. The three classes of integrons are similar in structure and are identified by where the insertions occur and what systems they coincide with. Class 1 integrons are seen in a diverse group of bacterial genomes and likely are all descendant from one common ancestor. The prevalence of the integron has shaped bacterial evolution by allowing rapid transfer of genes that are novel to an organism, such as antibiotic resistance genes.

Genetic engineering

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 ...

, a gene cassette is a manipulable fragment of DNA carrying, and capable of expressing, one or more genes of interest between one or more sets of restriction sites. It can be transferred from one DNA sequence (usually on a

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

) to another by 'cutting' the fragment out using

restriction enzyme A restriction enzyme, restriction endonuclease, REase, ENase or'' restrictase '' is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class ...

s and 'pasting' it back into the new context. The vectors containing the gene of interest typically also carry an antibiotic resistance gene called a

selectable marker 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, an ...

to easily identify cells that have successfully integrated the vector into their genome. To introduce a vector into a target cell, a state of competence must be inferred on the cell. This state is induced in the lab by incubating cells with calcium chloride before a brief heat shock, or by

electroporation Electroporation, or electropermeabilization, is a microbiology technique in which an electrical field is applied to cells in order to increase the permeability of the cell membrane, allowing chemicals, drugs, electrode arrays or DNA to be introd ...

. This makes the cells more susceptible to the plasmid that is being inserted. Once the plasmid has been added, the cells are grown in the presence of an antibiotic to confirm the uptake and expression of the new genetic elements. The usage of CRISPR/Cas9 systems has shown success in inserting genes into eukaryotic genomes. While CRISPR modification is still in its infancy, there is significant evidence for usage in combination with other techniques to produce high throughput (HTP) genome editing systems. Genetic engineering of bacteria for production of a variety of industrial products, including biofuels and specialty chemicals/nutraceuticals is a major area of research.

Horizontal gene transfer

Horizontal gene transfer (HGT) is the transfer of genetic elements between cells other than parental inheritance. HGT is responsible for much of the spread of antibiotic resistance among bacteria. Gene cassettes containing antibiotic resistance genes, or other virulence factors such as exotoxins, can be transferred from cell to cell via phage, transduction, taken up from the environment, transformation, or by bacterial conjugation. The ability to transfer gene cassettes between organisms has played a large role in the evolution of prokaryotes. Many commensal organisms, such as ''E. coli'', regularly harbor one or more gene cassettes that convey antibiotic resistance. Horizontal transfer of genetic elements from non-pathogenic commensals to unrelated species results in highly virulent pathogens that can carry multiple antibiotic resistance genes. The increasing prevalence of resistance creates challenging questions for researchers and physicians.

References

* * {{cite journal , last1 = Partridge , first1 = S , last2 = Tsafnat , first2 = G , last3 = Coiera , first3 = E , last4 = Iredell , first4 = J , year = 2009 , title = Gene cassettes and cassette arrays in mobile resistance integrons , journal = FEMS Microbiology Reviews , volume = 33 , issue = 4, pages = 757–784 , doi=10.1111/j.1574-6976.2009.00175.x, pmid = 19416365 , doi-access = free

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