Plasmids Flashcards
Describe some features of plasmids.
- Are extra-chromosomal DNA molecules that can be circular or linear
- Have autonomous replication- not dependant on host cell
- Control their own copy number (number of copies of plasmid per chromosome)
- Ensure inheritance at each cell division by a process called partitioning
What is incompatibility?
Describes how a plasmid cannot co-exist with a relative in the same cell (can’t have the same replication mechanism and proteins). If 2 plasmids are in the same incompatibility group, this problem can be solved by the plasmids going into different cells by directed partitioning, or one plasmid is ‘lost’.
What is the order of topoisomers of plasmids seen in gel electrophoresis, from least to most migration?
Open circular (one nick) -> linearised plasmids (two nicks) -> supercoiled plasmids
Do small or large plasmids tend to have a high copy number?
Small plasmids tend to have high copy numbers. Larger plasmids have lower copy numbers.
What type of partitioning do small plasmids tend to do?
Random plasmid partitioning (go into random daughter cells)
What type of partitioning do large plasmids tend to do?
Directed plasmid partitioning, in which the plasmids are directed into the daughter cells. This is important as there are fewer plasmids (as larger so lower copy number) so don’t want it to be random i.e. don’t want to have more than one in one cell, and don’t want it to get lost (want to spread as much as possible).
What does the term ‘promiscuous’ mean in relation to plasmids?
It means that the plasmid is broad host range (doesn’t have to replicate only in related species), and has the ability to transfer DNA by conjugation.
Name some characteristics that plasmids can encode.
- Antibiotic resistance
- Metal/metalloid resistance
- Virulence determinants
- Bacteriocin production
- Biodegradative capabilities
- Symbiotic determinants
What are the two plasmids that Bacillus anthracis virulence is dependant on? What do they encode and what happens when one is removed?
pXO1 plasmid encodes the anthrax toxin, and pXO2 encodes the proteins required fro capsule production. Loss ox pXO1 results in a loss of virulence. The pXO2 plasmid on its own can be used as a live attenuated vaccine as it doesn’t contain the gene for the toxin.
Explain how Agrobacterium tumefaciens can cause crown gall disease in plants.
If the bacteria contain the Ti plasmid, transfer DNA (t-DNA) from the plasmid can integrate into the host plant genome, causing the production of auxin, which leads to cell proliferation and tumours on the plants.
Explain the relationship between the pMRC01 plasmid and bacteriocin (antimicrobial agent).
The plasmid causes both the production of bacteriocin, and gives immunity to bacteria carrying it. Bacteria without the plasmid are killed by bacteriocin.
What allows transposons to move?
The ends of the transposons have inverted repeats that are recognised by transposases, which cut in these regions and allow the transposons to move. But can also get transposons that move by conjugation.
Why do the plasmid and the strain sometimes co-evolve?
To decrease the fitness cost to the strain (as having a plasmid incurs an energetic cost- requires more ATP and thus the plasmid must provide a fitness benefit).
What is an example of co-evolution?
The IncPbera plasmid has a mutation in the 5’ end of the TrfA1 gene, eliminating the fitness cost of the plasmid and resulting in improved fitness in Shewanella species.
What does the TrfA1 protein do when it does not have the N-terminus mutation?
It interacts with chromosomally encoded helicase which reduces the fitness of the host, as it prevents the helicase binding and interacting with the DNA and thus the replication of the chromosome is slowed.
