Team:Valencia Biocampus/Bacterium

From 2012.igem.org

Bacteria Subteam

Here is an overview of how our bacteria work. For more information look the molecular mechanisms below.

MOLECULAR MECHANISMS

LACTOSE-INDUCED PROMOTER

This construction is made up of three parts: (1) the transcription factor-binding site insidethe promoter, (2) the repressor-binding site outside the promoter and (3) the coding sequence, which contains a synthetic fluorescent (blue) protein. Our construction uses the well-known lactose operon system{1}. Since there is an operator region that blocks transcription, it is necessary to know it and avoid it.

When is the protein synthesized? In order to obtain the blue fluorescent protein two conditions have to be met. First condition: there is no glucose in the medium. Second condition: lactose is present in the medium (it also works with other inductors, like IPTG).

The molecular mechanism underlying this phenomenon is as follows: a lack of glucose promotes the formation of CAP (or CRP{2}), which binds to specific sites upstream of sugar-metabolizing genes and activates its transcription. The binding of this molecule depends on the presence of the allosteric effector cAMP (the concentration of this metabolite changes in response to the presence or absence of different nutrients).Moreover, another condition is needed since LacI repressor, produced constitutively by the lacI gene inside the lactose operon, will bind to the operator region and will block the transcription unless lactose is also present in the medium.Once lactose enters the cell it is converted to allolactose{3}, and this molecule binds tightly to the repressor so it can no longer block the transcription. Then, the fluorescent protein can glow in the cytoplasm!

¿How did we deal with this construction?In our experiments we not only tested the differential expression when glucose is absent + lactose present and vice versa, but we also tested the expression and growth rates when different compounds were added as carbon-enriched sources. For example, we added sodium acetate and galactose as substitutes of glucose. We determined the best IPTG concentration for our cultures too.

References:
1. F. Jacob and J. Monod. (1959) Genes of structure and genes of regulation in the biosynthesis of proteins. C. R. Hebd. Seances Acad. Sci.249: 1282–4.
2. S., Busby and R.H., Ebright. (2001). Transcription activation by catabolite activator protein (CAP). J. Mol. Biol.293: 199–213.
3. RE., Huber, K., Wallenfels and G.,Kurz. (1975)The action of beta-galactosidase (Escherichia coli) on allolactose.Canadian Journal of Biochemistry, 53(9):1035-1038