Selectable Marker

Definition: This element is required for the maintenance of the plasmid in the cell. Due to the presence of the selective marker, the plasmid becomes useful for the cell. Under the selective conditions, only cells that contain plasmids with the appropriate selectable marker can survive. Commonly, genes that confer resistance to various antibiotics are used as selective markers in cloning vectors. The drawbacks of this aproach are: 1. loss of selective pressure as a result of antibiotics degradation and inactivation. 2. contamination of the product or biomass by antibiotics, which may be unacceptable from medical or regulatory considerations.

Frequently Used Antibiotic Agents
(Mechanism of Action)
Experimental Tips for Antibiotics Use
Alternative Selection Strategies
Inspect Commercially Available Plasmids 

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Alternative Selection Strategies

The following reviews address this issue:

Frequently  Used Antibiotic Agents
Type of Cell
Mechanism of Action
Mechanism of  Resistant Gene
Binds to and inhibits a number of enzymes in the bacterial membrane that are involved in the synthesis of the cell wall.
The ampr gene on the plasmid codes for an enzyme that is secreted into the periplasmic space of the bacterium, where it catalyzes hydrolysis of the B-lactam ring of ampicillin, with concomitant detoxification of the drug.
Binds to the ribosomal 50S subunit and inhibits bacterial protein synthesis.
The camr gene encodes for a tetrameric, cytosolic protein (MW of each subunit 23,000) that in the presence of acetyl coenzyme A, catalyzes the formation of hydroxyl acetoxy derivatives of chloraphenicol that are unable to bind to the ribosomes.
Binds to ribosomal 70S subunit and inhibits bacterial protein synthesis.
Binds to a protein of the 30S subunit of the ribosome and inhibits ribosomal translocation 
The tetr gene encodes a membrane associated protein that prevents the antibiotics from entering the cell. 
Blasticidin S
Prokaryote & Eukaryote
Inhibits protein synthesis.
The blasticidinr gene encodes for a blasticidin deaminase, which converts blasticidin S to a non toxic deaminohydroxy.
Binds to the 30S and in some cases the 50S subunit causing miscoding; inhibits initiation and elongation during protein synthesis.
The neor gene encodes neomycin phosphotransferase which confers resistance to G418 (Geneticin), and the gene, encoding dihydropholate reductase (DHFR). When DHFR is used, the recipient cells must have a defective DHFR gene, which makes them unable to grow in the presence of methotrexate (MTX), unlike transfected cells with a functional DHFR gene
Hygromycin B
Inhibits protein synthesis by disrupting translocation.
The hygr gene - hygromycin B phosphotransferase (HPH) - 
Sigma's List of Antibiotic sorts several antibiotics according to their mechanism of action.

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Experimental Tips for Antibiotics Use

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