Question
Transposons.
Answer
Transposons (“jumping genes”) are mobile genetic elements — segments of DNA capable of moving from one location to another within a genome (from one site on the chromosome to another, from chromosome to plasmid, or between plasmids) — independent of the normal homologous recombination system, using their own encoded transposase enzyme.
Structure: typically flanked by inverted repeat sequences at each end, which are recognized by transposase to mediate excision/insertion; between these repeats, transposons can carry various genes, notably including antibiotic-resistance genes in many clinically important transposons.
Mechanism: transposase, encoded by the transposon itself, recognizes the inverted repeat sequences and catalyses excision of the transposon (in “cut-and-paste” transposition) or replication of the element (in “replicative” transposition), followed by insertion at a new target site, which does not require sequence homology between the transposon and the target site (distinguishing transposition from homologous recombination).
Clinical/biological significance:
- Spread and reassortment of antibiotic resistance genes — because transposons can move between plasmids and the chromosome, and between different plasmids, they allow resistance genes to be reshuffled and combined onto a single mobile element, and to move even between plasmids that are otherwise incompatible for direct co-existence in the same cell — significantly amplifying the diversity and mobility of resistance gene combinations beyond what plasmid transfer alone would achieve.
- Insertion of a transposon into a functional gene can disrupt gene function, sometimes used experimentally (transposon mutagenesis) to study gene function, and occurring naturally as a mechanism of genetic variation.
- Some transposons carry virulence genes in addition to/instead of resistance genes, contributing to pathogenicity.