Chapter 23: Plasmids

Plasmid

Self-replicating genetic elements sometimes found in both prokaryotic and eukaryotic cells, which are neither chromosomes nor part of the host cell's permanent genome.

Replicon

Molecule of DNA or RNA that contains an origin of replication and can self-replicate.

Copy number

The number of copies of a plasmid found within a single host cell.

Bacteriocin

Toxic protein made by bacteria to kill closely related bacteria.

ColE1 plasmid

Small multicopy plasmid that carries genes for colicins of the E group and is used as the basis of many widely used cloning vectors.

Colicin

Toxic protein or bacteriocin made by E. coli to kill closely related bacteria.

Incompatibility

The inability of two plasmids of the same family to co-exist in the same host cell.

Mobilization

Ability of a nontransferable plasmid to be moved from one host cell to another by a transferable plasmid.

Transferability

Ability of a plasmid to move itself from one host cell to another.

Bidirectional Plasmid Replication

A process where DNA is copied in two directions from a specific origin site on a plasmid.

Rolling Circle Replication

A process that replicates circular DNA or RNA molecules in a single direction.

Type I plasmid (pull)

Filaments drag one plasmid copy to each pole during cell division. ParABC

Type II plasmid (push)

Filaments elongate, pushing each plasmid copy to a pole during cell division.

Colicin E1

A bacteriocin produced by E. coli that kills other nearby E. coli cells by forming a pore in their outer membrane.

Colicin Immunity

The mechanism where a bacteria producing colicin E1 produces an immunity protein to prevent harm from its own colicin.

Ampicillin mode of action

Targets the cell wall by preventing cross-linking of peptidoglycan.

Chloramphenicol mode of action

Targets protein synthesis by blocking formation of the peptide bond.

Kanamycin / Neomycin mode of action

Targets protein synthesis by binding multiple sites on both the 30S and 50S ribosomes.

Tetracyclines mode of action

Targets protein synthesis by binding the 16S rRNA and the 30S subunit.

Ampicillin resistance mechanism

Bacterial β-lactamases inactivate β-lactam drugs.

Chloramphenicol resistance mechanism

Bacterial chloramphenicol acetyl transferase (CAT) inactivates the drug by adding 2 acetyl groups.

Kanamycin / Neomycin resistance mechanism

Neomycin phosphotransferase (encoded by npt gene) inactivates both kanamycin and neomycin.

Tetracyclines resistance mechanism

Tetracycline-specific efflux pumps (encoded by tetH) export the drug from the bacterial cell.

regulatory mechanisms for controlling plasmid copy number

negative feedback mechanism mediated by transcription of an antisense RNA from an alternative promoter

function of ribonuclease H

cleaves the RNA strand within a RNA-DNA hybrid molecule to generate a primer with a free 3’-OH group

essential properties of plasmid

have dsDNA, are supercoiled, extrachromosomal, can self-replicate, can be lost spontaneously or by curing agents, self-transferrable

5 types of bacterial plasmids

1. F-plasmids (fertility)

2. Col plasmids

3. Degrative plasmids

4. Virulence plasmids

5. R-plasmids (resistance)