Horizontal Gene Transfer

Horizontal Gene Transfer

• gene movement between cells that are not direct descendants 

  • allows quick acquisition of new characteristics 

  • fuels metabolic diversity

Three Mechanisms of HGT

• Transformation

• Transduction

• Conjugation

Three fates of HGT

• Degradation (no change)

• Replication by itself

• Recombination with host genome

Transformation

Free DNA is incorporated into a recipient cell and brings about genetic change

• Competent cells can take up DNA

• DNA can be added to the chromosome using RecA or a plasmid can be replicated in the cytoplasm without recombination

• Can lead to a stable transformation or the DNA can degrade, which leads to an unsuccessful transformation.

Transduction

Transfer of DNA from one cell to another by a bacteriophage (virus that infects bacteria)

• Low efficiency: 1/10^6 to 1/10^8 are transduced

• Can be generalized or specialized

Generalized transduction

• Non-specific DNA from the host genome is packaged inside the virion

  • Donor genes will be lost without homologous recombination

• Caused by the lytic cycle

  • Lytic: A virus is going to infect, replicate, and leave. Lytic cycle leads to the death of the host.

• A bacteriophage is going to infect a bacterium carrying out its normal viral replication cycle, which we'll talk about more specifically. Typically, that involves breaking up all of the host DNA because it doesn't need it, and then replicating its own DNA. And then packaging of this new virus typically occurs just randomly, the virus structure is going to form around pieces of DNA.

• Sometimes these viruses mistakenly package around bacterial DNA. When that new bacteria phage leaves it carries bacterial DNA, it can bind to and begin to infect a new host. And when it inserts DNA, it's the donor DNA and not viral DNA, thereby inserting linear DNA into a host. That linear DNA can then be incorporated into the chromosome by homologous recombination. So now you have successful transduction. 

Specialized transduction

• DNA from a specific region of the host chromosome is integrated directly into the virus genome

  • Donor genes inserted by homologous recombination or during viral replication

• Starts with lysogenic cycle

  • Lysogenic phages (integration of phage into the host genome)

• Normal:

  • Starts with induction. The phage DNA (prophage) circularizes and detaches from the host DNA

  • The detached DNA then replicates and once finished, the cell lyses and normal phages are released

• Rare:

  • Starts with induction. A portion of the host DNA is exchanged for phage DNA.

  • Detached DNA then replicates and once finished, the cell lyses and defective phages are released which can transduce the host DNA within.

Conjugation

• Mating

• Horizontal gene transfer that requires cell-to-cell contact

• Plasmid-encoded (F-plasmid)

• Requires relatedness

• Transfer DNA through pilus

• Donor cell: Contains conjugative plasmid (F+)

• Recipient cell: Does not contain plasmid (F-)

• F+ always mates with F-

• F+ and F+ don’t mate

Genetic Map of the F (Fertility) Plasmid of Escherichia coli

• oriV: Vertical gene transfer

• oriT: Horizontal gene transfer

F Plasmid DNA Conjugation

• Takes five minutes

• Beings with F+ and F-

• Ends with F+ and F+

• Donor and recipient come into physical contact

Hfr Strains

• F+: Cells possessing a nonintegrated F plasmid

• Hfr (high frequency of recombination): Cells possessing an integrated F plasmid

  • High rates of genetic recombination between genes on the donor (Hfr) and recipient (F-) chromosomes

  • Integration provides mechanism for mobilizing a genome

  • Cannot accept f plasmid from F+

  • Only mates with F- and allows F- to remain negative

• F- cell does not receive entire copy of the F plasmid instructions but products are still genetically different

F’ Conjugation

• F’ (F-prime) plasmids are F plasmids containing chromosomal genes

• F’ plasmids transfer chromosomal genes to recipients at high frequency

• Allows pair with F- cells

Deinococcus radiodurans

• Withstand a 1000x more radiation than a person can

• Humans die from 500-1000 rads

• Deinococcus radiodurans can survive up to a 1.5 million rads

• Most radiation resistant organism on the planet

• Can repair mutations in ss (single stranded) and dsDNA (double stranded)

• Cleans up toxic waste