Horizontal Gene Transfer

Horizontal gene transfer for higher education

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One feature of microbial genetics and perhaps the most striking genetic difference between humans and microorganisms, is the ability to not just pass genetic material from parent to progeny, but to pass DNA between two cells of the same generation.

Conjugation

Conjugation is the horizontal transfer of DNA from one cell to another directly. Conjugation is often mediated by cell-cell interactions through surface structures such as type 4 secretion systems (T4SS) and conjugative pilli. The genetic material transferred through conjugation are often circular pieces of DNA called plasmids. Plasmids are able to replicate autonomously within the cell, and also be combined into the bacterial genome through homologous recombination. The diversity and ease of manipulation of plasmids have made them a powerful biotechnological tool both in research and industry.

Plasmids have a wide variety of uses in research and industry for genetic engineering. They can be used to express large amounts of recombinant proteins (enzymes for example) as well as deleting genes to determine their function.

The ability of plasmids to replicate independent of the host chromosome, is mediated through a sequence known as the origin of replication. This sequence can be recognised by the host machinery, recruiting it for plasmid replication. Plasmids encode a diverse array of proteins with different functions; the best studied of which are those conferring increased virulence, or resistance to antibiotics. There are many proteins involved in the maintenance and transfer of plasmids, but surprisingly little is known about their mechanistic roles.

Transformation

Transformation (in the context of HGT) is the direct uptake of exogenous DNA by bacteria from their environment. Exogenous DNA often comes from the death and lysis of other cells, followed by release of the genetic material.

Roughly 1% of bacteria are capable transformation naturally. Most species need to be induced into a state of competence whereby DNA uptake is far more likely. Competency for transformation can be induced either chemically (chemo-competent) or by subjecting the cells to an electric current (electro-competence). Once inside the bacteria cell, exogenous DNA can be incorporated into the host chromosome through homologous recombination.

Transduction

Transduction is the transfer of DNA horizontally from one bacterial cell to another via a bacteriophage (phage) intermediate. Phage are viruses that specifically infect bacteria, first discovered independently by English bacteriologist Frederick Twort, and French-Canadian microbiologist  Félix d'Hérel. Phage have two distinct life cycles; lytic and lysogenic.

To express their genes and replicate their genetic material, phage must hijack the host cell machinery. Upon recruitment of the host machinery, phage coat particles will be expressed, and replicated genomes packed into them. The number of phage in the cell will eventually reach breaking point, splitting the cell open and releasing phage particles. This is the lytic cycle.

Phage can also integrate their genomes with the host chromosome, forming a prophage. This is known as a state of lysogeny. Once integrated, the prophage can remain in the bacteria genome indefinitely, until they are exposed to a certain stimuli, causing excision of the prophage, and passage into the lytic cycle. Excision and packaging of the phage genome is an error-prone process, meaning that bacterial DNA can occasionally be packed into the phage particles. When these new phage are released following bacterial lysis, they are free to infect new bacterial cells, taking their previous hosts' DNA with them.

You can read more about phage biology on the bacteriophage page.

The diagram above shows the process of DNA transduction following induction of the phage lytic life cycle.

 

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