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Horizontal gene transfer
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Also known as lateral gene transfer
From one organism to another
Acquisition of exogenous DNA by prokaryotes
Bacteria better understood than archaea
Three methods depending on DNA source
->Transformation
->conjugation
->transduction
• At least one of the methods can be observed in nearly all bacteria
• Important role in evolution
• Traits involved NOT transferred by inheritance from one generation to the next
• Instead introduced from unrelated individuals or from different species
• Good for gene mapping and constructing bacterial strains to test function and regulation
of specific genes
• DNA transfer asymmetrical in 2 ways
->transfer is unidirectional – donor to recipient
->most recipients receive 3% or less of donor DNA
Why prokaryotes transfer genes
• Enables rapid adaptation of prokaryotes to new environments
->huge impact on human health and environment
->acquire resistances to anti-biotics or heavy metals
• Contributes to the evolution of the prokaryotic genome
->source of variation
->faster than waiting for mutations to accumulate
Donor DNA
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Bacterial uptake has a variety of sources
Donor bacterium
->plasmids (most common) = small, circular, extra-chromosomal
->insertion sequences = small mobile elements that contain transposition genes with repeats at
ends
->transposons = insertion sequence but larger, containing other genes (anti-microbial/heavy metal
resistance) may integrate into and excise from host chomosome/plasmids
• Virus
->transducing phages carry bacterial DNA between cells
• Naked DNA
->from lysed donor cell
Transformation – discovery – 1928, confirmation - 1944
• DNA fragment/plasmid taken up from environment
->originally from lysed donor
->can be any of DNA forms above
->up to tens of kbs transferred
->can occur across species and rarely across kingdoms
• Initiated by recipient cell
->recipient cell known as the transformant
• Similar process in eukaryotes = transfection

Transformation mechanism
• Competence developed
• DNA binds to cell surface receptors
• Free DNA processed by cell surface enzymes
->nicked, digested by endonuclease, one strand cleaved by exonuclease, single strand protected by
binding proteins
• DNA is taken up through protein/poly hydroxybutyrate-calium polyphosphate
translocation channel
• DNA integrated by recombination in recipient (requires RecA = protein essential for the
repair and maintenance of DNA)

Conjugation – discovery – 1946
• Transfer of DNA from donor to recipient via direct contact through conjugation pilus
->recipients known as trans/ex- conjugants
• Broadest range of all three HGT methods
->been demonstrated between bacteria and plants
• DNA transferred may be plasmid, part of a chromosome or other transmissible
elements
->can be large, up to thousands of kb

->fertility plasmid most widely studied
• Requires the presence of genes for conjugation
->may be transferred as conjugative plasmids or conjugative transposons

Transduction – discovery – 1952
• Bacterial DNA transferred between donor and recipient via a virus
->virus is called a vector in this case
->virus is a bacteriophage or phage
->up to hundreds of kb transferred
• Phages very specific to cell surface receptors
->narrowest host range of three HGT methods
• Recipients known as transductants
• 2 forms – GENERALISED and SPECIALISED
• 2 forms of phage
->lytic – infect cells, immediately causing them lyse
->lysogenic – infects cells and integrates into plasmid

Summary
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3 methods of HGT
Discovered in the first half of the 20th century
DNA transferred from donor to recipient
HGT contributed to evolution of antimicrobial resistance, metabolic flexibility, virulence
and variation in bacteria
HGT can be exploited by molecular biologists wishing to make new genetically modified
organisms



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