Alternative selectable and counter-selectable markers:


Genetic modification requires us to discriminate between bacteria which have taken up the DNA of interest and those which have not. This is traditionally done by using antibiotic resistance markers – cells that have taken up the DNA of interest (along with these markers) will be able to survive on media supplemented with the relevant antibiotic while those that do not have the DNA will not grow.

Selectable markers select for the cells which have taken up the gene of interest (eg. sucrose hydrolase) while counter-selectable markers select against the cells which have the DNA of interest (nitroreductase and SacB), which may be useful if we want to get rid of the cells that still contain a no longer wanted DNA insert.

The problem with this system is that international law does not allow the release of genetically modified organisms which contain such antibiotic resistance markers because these might aid the spreading of antibiotic resistance genes in the wild population.

We have questioned the legacy and safety of using antibiotics for selection and counter-selection and thus we aim to provide alternative markers that do not necessarily rely on antibiotic resistance genes for selection or counterselection. This would allow iGEM projects (and other projects where genetic engineering is used) to pass this hurdle on their way to releasing their constructs into the environment where they could be used for the real life purpose they were built for. This does not, of course, means that any constructs can now be released into the wild, as the properties of the construct still need to be considered, but that the issue of antibiotic resistance will no longer be present.


Methods (expand)

Inserting gene into a biobrick vecor: Cloning a PCR product into a biobrick vector protocol on OpenWetWare ( however NEB buffers were used.

DNA gel preparation: Analysing DNA by gel electrophoresis protocol on OpanWetWare ( however 0.5*TAE rather than 1*TAE was used.

Colony PCR screen: Screening colonies by PCR protocol on OpenWetWare

Transformations: Preparing and using compenent E.coli cells protocol on OpenWetWare (

PCR reactions : Cloning parts by PCR with Kod polymerase protocol on OpenWetWare (

Minipreps : Plasmid DNA minipreps from Escerichia coli JM109 and similar strains protocol on OpenWetWare (

Digests to linearise the DNA frangment/determine size of insert: Analytical restriction digests protocol on OpenWetWare (

DNA purification: Purifying a PCR product from solution protocol on OpenWetWare ( however 165 ul NaI, 5 ul glass beads,180 ul wash buffer and 10 ul EB were used.

DNA preparation for sequencing: 2.5 ul miniprepped DNA, 2 ul water and 1 ul forward primer ( specific for biobrick prefix) or reverse primer (specific for biobrick suffix)

Nitroreductase activity assay: Overnight liquid cultures of nitroreductase strains were centrifuged at 10000 rpm for 5 mins to pellet the cells. The cells were then resuspended in 250 ul PBS and 1 ul DTT to ensure that cellular proteins are not oxidized. The solution was sonicated 6* (10 s sonication+20 s rest). The supernatant was separated from the pellet by centrifugation and used for the NADH-dependent nitroreductase activity assay.

To assess background activity NADH (5 ul) and bacterial supernatant (5 ul) were added to 0.8 ml PBS and mixed. OD340 was measured for 1 minute. DNBA(5 ul) was added to the same cuvette to start the reaction and change in OD340 was monitored for 1 minute. DMSO(5 ul) was used a control (DNBA is dissolved in DMSO)

The protein concentration of each of the supernants was estimated by by Bradford protein assay using the Pierce reagent protocol on OpenWetWare(

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Works Cited (expand)

French, C., & Kowal, M. (2010, 09 24). B. subtilis levansucrase. Lethal to E.coli in presence of sucrose. Retrieved 2012, from Registry of standard biological parts:

Gay, P., Coq, D. l., Strinmetz, M., Ferrari, E., & Hoch, J. A. (1983). Cloning Structural Gene SacB, which Codes for Exoenzyme Levansucrase of Bacillus subtilis: Expression of the Gene in Esherichia coli. Journal of Bacteriology , 1424-1431.

Jahreis, K., Bentler, L., Bockmann, J., Hans, S., Meyer, A., Siepelmeyer, J., et al. (2002). Adaptation of sucrose metabolism in the Escherichia coli Wild-Type Strain EC31132. Journal of Bacteriology, 5307-5316.

Keuning, S., Janssen, D. B., & Witholt, B. (1985). Purification and Characterisation of Hyrdrolytic Haloalkane Dehalogenase from Xanthobacter autotrophicus GJ10. Journal of Bacteriology, 635-639.

Naested, H., Fennema, M., Hao, L., Andersen, M., Janssen, D. B., & Mundy, J. (1999). A bacterial haloalkane dehalogenase gene as a negative selectable marker in Arabidopsis. The Plant Journal, 571-576.

Nicklin, C. E., & Bruce, N. C. (1998). Aerobic degradation of 2,4,6-Trinitrotoluene by Enterobacter cloaceae PB2 and by Pentaerythritol tetranitrate reductase. Applied and environmental microbiology , 2864-2868.

Nillius, D., Muller, J., & Muller, N. (2011). Nitroreductase (GlNR1) increases susceptibility of Giardia lamblia and Escherichia coli to nitro drugs. Journal of antimicrobial chemotherapy, 1029-1035.

Kang et al. (2009). "Levan: Applications and Perspectives". Microbial Production of Biopolymers and Polymer Precursors. Caister Academic Press

Dahech, I, Belghith, K. S., Hamden, K., Feki, A., Belghith, H. and Mejdoub, H. (2011) Antidiabetic activity of levan polysaccharide in alloxan-induced diabetic rats. International Journal of Biological Macromolecules 49(4):742-746

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