Multimer resultion failure
- two plasmids recombine into one big one
- means one cell wont have a plasmid
Avoidance of oligomerisation: multimer resolution
Problem: plasmids multimers form by recombination
- bigger chance of plasmid loss during cell division
Solution: plasmid-encoded site
Two examples of avoidance of oligomerisation: multimer resolution
- F encodes two site-specific systems
- ResD: recombinase - acts at fcr near oriV
- Tn1000: has own system
- ColE1: uses host-encoded recombination: has 35-bp cer site for site-specific recombination by XerCD (chromosomal-encoded)
(- takes one dimer and lines up the two sites and enzyme them forms two monomers)
Multimer resultion - ColE1
- ColE1: uses host-encoded recombination: has 35-bp cer site for site-specific recombination by XerCD (chromosomal-encoded)
(- takes one dimer and lines up the two sites and enzyme them forms two monomers)
What is the problem of having a plasmid-free cell
- plasmid-free segregants out-grow plasmid-bearing cells since the fitness costs of plasmids can slow bacterial growth
How do you control plasmid free cells?
Kill plasmid-free segregants through post-segregational killing systems
- two components
1) stable toxin
2) unstable antitoxin (antidote) - if plasmid does not enter cell, antidote is degraded and then the target is inactivated by toxin leading to cel death
Control of plasmid free cells in high Copt vs low copy
High copy ColE1
- cells which do not proudnce the immunity protein to ColicinE1 will die
Low copy F
- encodes at lease two host-killing toxin-antitoxin (TA
Control of plasmid free cells: Hok/sok system
(Two genes being encoded on opposite sides of the DNA)
Hok is much more stable but Sok is produced at a much faster rate
Because they are complementary they can base pair and trigger degradation
In a sense of the plasmid, HOK will remain present but Sok will rapidly degrade thus killing the cell
Hok: host killing - toxin
- small killer peptide (membrane depolarisation)
- Hok translated from stable messenger RNA (half-life 20 mins)
- much more stable
Sok: surpressor of host-killing - antitoxin
- unstable antisense RNA (half life of 5 min)
- binds to Hok mRNA and prevents it from being translated
Plasmid spread via conjugation - high copy vs low copy
(High copy) Conjugal plasmids - F
Tra and OriT
- tra genes encode mating pore and DNA mobilisation functions for conjugative transfer
- oriT is where DNA nicked and transferred
(Low copy) Mobilisation plasmids - ColEI
Mob and oriT
- mob is a relaxase required for mobilisation (acts at oriT)
- oriT is an origin of conjunctive transfer
- lacks tra genes but can use those from a conjugal plasmid if it is in the same cell
- can’t form pilus or mating complex without using F plasmid machinery (must be in same cell) - low energy
The accessory genome, plasmid summary
What are mobile genetic elements (MGEs)
- the agents of horizontals gene transfer
- contributors to microbial diversity and evolution through gene acquisition
Which are more abundant? Bacteria or bacteriophages?
Bacteria are outnumbered by a factor of 10 to 1 by pages that infect them
Facts about bacteriophages
- more bacteriophages then all other organisms in the world
- key role in carbon cycling
- kill 40% of ocean bacteria every day
- most current lab tools are derived from phage research
What is a temperate bacteriophage
- a pahge that can go into two different life cycles
Two life styles of a temperate bacteriophage
- lysogenic
- lytic
Temperate bacteriophage in the lysogenic pathway - intergration process
Lambda intergration is a process that requires
- attP site on the phage
- attB site on the bacterial chromosome
- lambda integrate
Why are temperate phages (intergration) important to bacterial evolution
They can encode virulence determinants
Lysogenic conversion: expression during lysogeny
- prophage encoding a toxic protein, exploiting normal bacterial excretion systems - it’s the pages that produce the virulence factors that are important for the emulation of specific pathogens
Lysogenic conversion: lystic subpopulation
- require lysis of host to release toxin
- kills cell but helps neither produce toxins and survive and succeed
Pathogenicity definition
The ability of a bacterium to cause disease
Virulence definition
The degree of pathogenicity of strains
Virulence vs pathogenicity definitions
Pathogenicity - the ability of a bacterium to cause disease
Virulence - the degree of pathogenicity of strains
Example of phases that encode virulence determinants
Features of lysogenic conversion
- is effeictient
- doesn’t require cell to cell contact
- incorporation is not homology dependent
Features of phages that make them really good
- can act as a population level - many pages released and convert numbers of the population
- phages can survive harsh conditions that eliminate bacteria
Features of phages
- temperate phages can undergo lysogenic lifecycle
- integrate into bacterial chromosome
- can carry virulence factors / toxins -> lysogenic conversion
- important for bacterial evolution
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