Lecture 11 Flashcards
What is a core genome?
- The essential bacterial genes on a chromosome
- Genes are conserved in all members of a species
- e.g. replication, cell membrane formation
What is an accessory genome?
- Located on a plasmid, transposon, chromosomal islands
- Non-essential genes of variable occurrence in species
- Acquired through horizontal transmission
What kind of genetic element is a plasmid?
An extrachromosomal genetic element
What do plasmids contribute to?
- Bacterial evolution
- Genetic plasticity
Are plasmids capable of autonomous replication?
Yes
What do plasmids with AbR genes encode?
Enzymes that modify and degrade antibiotics
What are AbR genes on plasmids carried on?
Transposons
What functions must a plasmid carry out for survival?
- Replicate
- Segregate = ensure each daughter cell receives at least one copy upon division
- Keep host happy
- constrain metabolic load by regulating copy number - Keep host controlled = kill of cells that kick them out
- Spread = via conjugation
How do Large Plasmids regulate copy number?
They have a low copy number, only have 1-5 copies per cell
How do Small Plasmids regulate copy number?
They have a high copy number, about 10-50 copies per cell
How do low copy plasmids partition plasmids at cell division?
Use specialised molecular machinery to ensure each daughter cell gets a plasmid
How do high copy plasmids partition plasmids at cell division?
Use random partitioning, as long as copy number is 15< the chance of non-inheritance is reduced
How is the Par M system (low copy plasmids) accomplished?
By two proteins (Par M and R) and a centromere-like DNA site (par s)
What is Par M?
An actin-like protein that polymerases to form filaments
What is Par R?
DNA binding adaptar protein
What is Par s?
A centromere-like region
Describe the steps in the Par M system
- Replicated plasmids are paired by Par R bound to Par s, forming a partitioning complex
- Complex forms a nucleation point for Par M filament
- Continuous addition of Par M to filament poles provides force for movement of plasmids to opposite cell poles
- When plasmids reach the poles, the filament depolymerises
(1. Replication, 2. Search/Capture, 3. Elongation, 4. Depolymerisation)
What is plasmid DNA replication controlled by?
By plasmid encoded inhibitor that acts on the OriV
In plasmid DNA replication, as cell size increases what happens?
The inhibitor concentration decreases and plasmid replication is initiated
In plasmid DNA replication, replication results in….
Further copies of the inhibitor gene and more inhibitor which limits plasmid replication again
What do incompatible plasmids share?
The same regulatory mechanisms and are subject to each other’s inhibitor
->one eventually gains upper hand and eliminates the other
What does the rom (rop) protein encode?
Encodes a protein that regulates plasmid copy number
Describe the control of plasmid replication in ColE1
- It’s mediated by plasmid-encoded anti-sense RNA
1. RNA II binds to ColE1 OriV to initiate replication
2. In presence of Rop protein, RNA I binds to RNA II and prevents it binding to OriV - > REPLICATION STOPS
- Formation of a persistant RNA II / DNA Hybrid is required for initiation of replication
- >’persistant’ because the RNA stays in the cell for a while, instead of leaving - At critical Rop concentration a dimer is formed
- >secondary RNA structure (G’s) allows it to form the structure and attach the DNA
What is the control of Plasmid replication in F mediated by?
A protein binding to repeated sequences
In the control of plasmid rep (F) what happens at low concentrations of Rep A?
Rep A binds to OriV and initiates replication
In the control of plasmid rep (F) what happens at high concentrations of Rep A?
Rep A also binds to iteron sequences and ‘hand-cuffs’ plasmids together this prevents replication until plasmids are separated at cell division
