Lecture 9 Flashcards
What is horizontal gene transfer and what does it contribute?
- when organisms acquire genes directly from another cell and incorporates them into its gene
- responsible for the spread of fitness exchanging traits, including antibiotic resistance
- provides a mechanism for ongoing adaptive evolution
What are mobile genetic elements? MGES
- the agents of horizontal gene transfer
- contributors to microbial diversity and evolution through gene acquisition
Examples of MGEs
- plasmids
- transpoons
- bacteriophages
-ICES ARE NOT COVERED
Core genome vs accessory genome
What makes up the pangenome
Core genome +accesory genome
Core genome + accessory genome =
Pangenome
Plasmid learning objectives - be able to answer thos
What are plasmids?
Extra chromosomal genetic elements
- capable of autonomous replication
- not essential to cell under all circumstances- not needed for day to day survival
- can contribute to bacterial evolution and genetic plasticity
- very important in recombinant DNA technology
- can encode important phenotypes
Different different plasmids that encode important phenotypes
What encodes for antibiotic resistance ?
Plasmids with AbR genes - R plasmids
Antibiotic resistant plasmids (R plasmids) features
- encodes proteins that provide resistance to antibiotics
- AbR genes on plasmids often carried on transpoons
- significant medically
Different phenotypes that are encoded by plasmids
Are plasmids useful to their host
Hmmm most plasmids are essentially molecular parasites as they are often of no use to their host
5 things a plasmid must do
• Replicate
– Large plasmids typically only 1-5 copies/cell (low copy number)
– Small plasmids ~15-50 copies/cell (high copy number)
• Keep host happy
– by constraining metabolic load by regulating copy number (enough copies to ensure it is inherited but not too many to overwhelm the host)
• Segregate
– ensure daughter cells receive at least one copy upon division
• Keep host under control
– kill off cells that lose the plasmid
• Spread
– conjugation!; non-conjugative plasmids are often mobilisable
Plasmids must maintain themselves in the population by ____, ____\______ and resolving
Plasmids must maintain themselves in the population by replication, partitioning/segregation and resolving multimers etc…
General priniclpke of the control of plasmid replication
~determines plasmid copy number
– Plasmid DNA replication controlled by plasmid-
encoded inhibitor that acts at oriV
– As cell size increases, inhibitor concentration decreases and plasmid replication initiated
– Replication results in further copies of inhibitor gene
and more inhibitor which limits plasmid replication
again
Replication of a high copy plasmid - ColE1
- oriV origin of “vegetative” replication is where DNA replication begins. Copy number maintained at 15
- oriV encodes 2 different non- coding RNAs and the DNA origin of replication
- rom inhibitor protein required for copy number control
RNA II binds to ColE1 oriV to initiate replication.
In presence of Rop (Rom) protein, RNA I binds to RNA II and prevents it binding to oriV
—-> replication stops
Hence concentration of RNA I and Rop protein in cell is critical.
The role of RNA II and RNA I in high-copy plasmid replication
RNA II binds to ColE1 oriV to initiate replication.
In presence of Rop (Rom) protein, RNA I binds to RNA II and prevents it binding to oriV à replication stops
Hence concentration of RNA I and Rop protein in cell is critical.
Is RNA 1 coding
No
What does ROP protein help do?
Bind
RNA 1 and 2
Low copy plasmid replication: F - what is RepF1A
What is High-copy plasmid replication mediated by?
a plasmid-encoded anti-sense RNA
What is Low copy plasmid replication mediated by?
a protein binding to repeated sequences (iterons):
Replication mediation of low copy plasmid
At low concentrations RepA binds to oriV and initiates replication.
At high concentration, RepA also binds to iteron sequences and “hand-cuffs” plasmids together, preventing replication until plasmids are separated at cell division.
Partitioning of plasmids at cell division - HIGH COPY PLASMIDS
- replicate to make at least 20 copies - up to 100s
- random partitioning
- porivided that copy number is maintained at 15 or greater then there is less then a 1 in 1 million chance of not being inherited
Partitioning of plasmids at cell division - LOW COPY PLASMODS
- require specialised molecular machinery as they are very large and only a couple per cell
- active partitioning
- machinery recognises them and pushes them to two ends of the cell - e.g type 1 - ParA and type 2 ParM
( - if plasmids are large can’t make many many copies without energy cost to the host)
Partitioning low copy plasmids - F
RepF1A - determines vegetative replication and incompatibility properties; includes oriV - allow 1-2 plasmid copies per cell
(these two now need to be pushed apart in the cell)
par: partitioning loci ensures they are apart
Partitioning of low copy plasmids - ParM system - what is it accomplished by
Partitioning is accomplished by 2 proteins (ParM and R) and a centromere-like DNA site (parS)
ParM System of partitioning low copy plasmids
- plasmids are paired by ParR bound to parS, thereby forming a partitioning complex.
- partitioning complex forms a nucleation point for ParM filamentaion
- addition of ParM to the filament poles forces movement of plasmids to opposite cell poles
- when the plasmids reach the poles the filament depolymerises
What is ParM?
An actin-like protein that can self polymerise to form filaments
(Can only do this after recognising ParR)
What is ParR?
A DNA-binding adaptor protein. parS is a controversial-like region
