Horizontal Gene Transfer Flashcards
What is meant by horizontal gene transfer?
The movement of genetic material horizontally between organisms, as opposed to vertical transfer between parents and their offspring.
What is meant by transformation of bacteria?
This is when a bacterial cell lysis releases its contents into the surrounding environment, where nearby competent bacteria can take up the genetic material released.
What is meant by transduction of bacteria?
This is when the genetic material is introduced into a bacteria through a virus or a virus vector. For example, DNA can be injected into bacteria from a bacteriophage and integrated into its own genome.
What is meant by conjugation of bacteria?
Conjugation is the process of which a bacterial donor directly transfers genetic material to a recipient through contact - the donor and recipient are therefore called a ‘mating pair’. Bacteria and archaea are both able to conjugate.
What are the two types of conjugative elements?
Integrative and conjugative element (ICE) is integrated into the host cell genome, whereas conjugated plasmid exist separately from the main genome and can autonomously replicate.
Describe the Type IV secretion system and its role in pilus biogenesis during conjugation.
The type IV secretion system (T4SS) is a transmembrane
protein complex. It forms the base of the pilus, and is where polymerisation and depolymerisation of the pilus subunits - pilin - occurs.
Describe the steps of pilus biogenesis.
The pilus consists of small subunits, called pilin, which polyermises together in a helical structure to form the pilus structure. The steps include:
1: Pro-pilin, the precursor for pilin, inserts into the membrane.
2: Pro-pilin is processed via post-translational modifications.
3: Pilin attaches to a phospholipid to form a mature pilin-phospholipid complex.
4: This then enters the T4SS and polymerises with other pilins to elongate the helical pilus structure.
5: The pilus constracts when it becomes into contact with another bacterium, in order to pull the two cells close together in preparation for mating pair formation.
How does conjugation differ between different conjugative systems?
Conjugative systems differ by what they are transferring during conjugation. For example, assembly of the F pilus is encoded for by the F plasmid, which then transfer a copy of itself during conjugation.
Different conjugative systems have different host ranges. For example, F pilus has a narrow host range of E. coli and other closely related species, whereas the RP4 pilus (called P pilus) has a wider host range of most Gram-negative bacteria.
Describe the features and functions of the mating pair formation (Mpf) system.
The Mpf system functions as a secretory machinery for intercellular genetic transfer. It consists of a minimal set of 10 conserved proteins, which forms a membrane-spanning protein complex and a surface-exposed sex pilus. These form to establish the intimate interactions between the mating pair. Mpf conjugative system just describes the T4SS during pairing.
What does it mean when plasmids are described as conjugative, mobilisable or non-mobilisable in the context of conjugation?
Conjugative plasmids encode the necessary proteins needed for conjugation, e.g. T4SS, within its own genome. Therefore, whatever host it is inhabiting, it can conjugate.
Mobilisable plasmids are within host cells that encode the conjugation system within the cellular genome. Therefore, plasmids can be conjugated when in these cells.
Non-mobilisable plasmids cannot conjugate because it does not have an environment with a conjugative system.
Describe the relaxosome and the relaxosome formation step of the conjugation process.
After mating pair formation, the relaxosome forms. This consists of an enzyme relaxase, oriT of the DNA, and many accessory proteins (differs between conjugative systems).
The accessory proteins bind to the relaxase and guide it towards the oriT sequence in the DNA substrate. When bound, the DNA is nicked on one strand by a trans-esterase domain of relaxase and it begins to unwind by the helicase domain. Accessory proteins then bind to stabilise this complex.
A coupling protein (CP) then bridges the relaxosome with the T4SS. CP is also responsible for switching the T4SS function from pilus biogenesis to DNA transfer.
For example, in an F plasmid relaxosome, IHF is a essential part of the relaxosome and is encoded by E. coli. This is why F plasmid conjugative is specific to E.coli because this IHF protein is necessary.
Describe the process of DNA transfer during conjugation.
Before DNA transfer, the CP sits outside of the T4SS structure, allowing pilin to insert into it to form the pilus. However, when the CP associates with relaxase (which is attached to the DNA), CP inserts itself into the T4SS structure and allows the relaxosome to pass through the structure into the donor. Only one strand of the DNA is fed through the T4SS into the recipient as the relaxase only nicked the DNA on one strand.
Explain the process of complementary strand synthesis during conjugation.
As the ssDNA is being transferred, the donor cell immediately starts to synthesise the complementary strand for the plasmid as it being fed through to the recipient. As the recipient recieves it, it also starts to immediately synthesise the ds. The recipient synthesises the new strand in small fragments because the DNA is being fed through with the 3’ leading.
Once complete, the two cells will now have a copy of the dsDNA each.
How does the conjugation process differ for ICE (integrative conjugative elements) as opposed to plasmids?
The ICE integrates into the host cell genome. Therefore, before the DNA transfer to T4SS, the ICE must excise itself from the chromosome and become an independent circular DNA.
After transfer of the DNA, the ICE then integrates itself into the host cell genome.
What happens when the transfer of DNA is initiated without ICE excising from the chromosome? Give Hfr E.coli as an example.
Hfr E.coli is a strain that have the F plasmid integrated within its genome. When conjugation of Hfr occurs to F- E.coli, the chromosome itself begins to be transferred. However, conjugation does not last long and not long enough for the whole chromosome to be transferred so it abruptly cuts at a random location.
This piece of DNA can either be degraded in the new host cell, or the conjugated DNA undergoes homologous recombination with the host cell chromosome.
