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Microbial Genetics (Micr340)
Lecture 7Plasmids
Plasmids
DNA molecules other than chromosomes
Widely present in most bacteria Roles: adaptation, evolution,
pathogenesis Usually encode genes not essential Mostly circular Sizes: a few kb to >100 kb
Naming Plasmids
Many name after genes they carry or functions of the plasmids
In the beginning: ColE1, producing Bacteriocin to kill E. coli Tol, degradation of toluene Ti, Tumor initiation in plants RK2, resistance to amp, tet, kanamycin
Now: pXXNNNN. eg, pHX200
Plasmidstructure
Plasmid purification
Plasmid purification
Plasmid replication
Replicons: DNA molecules that can replicate autonomously in cell (chromosome, plasmid)
All replicons have at least one origin of replication (ori)
For plasmid ori, it is called oriV (vegetative) Two mechanisms of replication:
Theta replication Rolling-circle replication
Theta replication
Rolling-circle replication
Function of ori region
Replication related genes close to ori. Host range determination
Narrow host range Broad host range
Regulation of copy-number Relaxed plasmids – high copy-number Stringent plasmids – low copy-number
Regulation of replication: ColE1 plasmids
Random plasmid curing
Plasmid Partition
Plasmid avoid being lost from dividing cells by carrying partitioning systems
The functions involved in these systems are called par functions
cis-acting site: parS trans-acting site: parA and parB
Incompatibility
Many bacteria contain multiple types of plasmids and can coexist for many generations
Not all types of plasmids can stably coexist in same cell; some interfere with each other’s replication or partition and one of them will be lost
This is called plasmid incompatibility
Incompatibility
Two plasmids that cannot stably coexist are members of the same incompatibility (Inc) group
If two plasmids can stably coexist, they belong to different Inc group
Coexistence of different Inc groups
Coexistence of same Inc groups
Measuring curing of a plasmid
Finding the ori of a plasmid