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