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12. Beskrive transposoner og deres forbindelse til antibiotika resistens
Devlin, s.190-1
Devlin, s.345-6, cc. 8.1

Transposition – movement of specific pieces of DNA in the genome.

Transposons – some pieces of DNA, that have two key features that enable them to move nearly anywhere into a target chromosome:

• They encode transposase enzymes

• They have insertion sequences that are recognised by the transposase, usually short inverted terminal repeat sequences, that are also used to define the transposon

Transposase catalyses the process of excision from one place in the genome and insertion into another; also replicative insertion into a new place without loosing the original piece of DNA.

Transposons can integrate into other chromosomes and move to new places within a chromosome. A significant fraction of the human genome has resulted from accumulation of transposons and insertion sequences.  

Transposons vary tremendously in length; they can consist of few thousand base pairs and contain two or three genes or many thousand base pairs and several genes.

Besides the transposase enzymes, transposons can also code for:

• Toxins

• Proteins that hydrolyse antibiotics. Transposons contain antibiotic resistance sequences and therefore have a key role in antibiotic resistance.

Plasmids - independent DNA molecules found in bacteria, contain genes that facilitate their transfer from one bacterium to another. Plasmids can contain transposons incorporated in their DNA.

As the plasmids transfer, e.g. between different infecting bacterial strains, the transposons containing antibiotic resistance genes are moved into the new bacterial strain. Once inside a new bacterium, the transposon can duplicate onto the chromosome and become permanently established in the cell’s lineage. The result is that more and more pathogenic bacterial strains have become resistant to increasing number of antibiotics.

Many cases have been documented in which a bacterial strain in a patient being treated with one antibiotic suddenly becomes resistant to that antibiotic, and simultaneously, to several other antibiotics, even though the bacterial strain had never previously been exposed to these other antibiotics. 

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