Lighting genes I

by Bio! Mexico


Lighting genes I
Background of light induction in Synthetic Biology


Chemical inductors and osmolarity or temperature variations are commonly employed for the induction of recombinant proteins expression. But this kind of stimuli are invasive and may elicit other responses in the cell; furthermore, as in the case of chemical inductors, the cost of the reactants can not be discarded at the industrial scale

Where can we find a specific, non-invasive and low-cost stimulus?


The answer that Shimizu-Sato and collegues found in 2002 was a very interesting and elegant one: light.

They used phytocromes procedent from the plant Arabidopsis thaliana and they adapted them to a double-hybrid assay in a yeast chassis, in such a way that when red light was present, a LacZ reporter (measured in Miller units) was expressed; meanwhile, when far red light was present, the LacZ reporter expression was repressed. Nevertheless, the main issue with the procedure was that exogenous chromophore -the chemical needed for light reception- addition in the culture medium was required.

A few years later, in late 2005, the team of Dr. Voigt published a brief communication that would turn into a cornerstone of Synthetic Biology: Engineering Escherichia coli to see light.

Bacterial photography. From Levskaya, et al., (2005)
The authors describe how they built a chimeric protein, using domains from the E. coli EnvZ osmoregulator and the Synechocystis Cph1 phytochrome. They coupled the transcription response of a LacZ reporter to this chimeric photoreceptor, so at high luminosity, the LacZ was poorly expressed, but at low luminosity, the LacZ reporter was highly expressed. This LacZ reporter was monitored through the presence of a black precipitate, so in a word, they have turned culture plaques into black-and-white photographs! Furthermore, they also made the bacteria synthesize their own chromophore molecules, so their addition to the culture medium was no longer needed. The development of this “bio-photography” techniques has found many followers, like the people of  BioBuilder from SynBERC, who with some beautiful results have shown a glimpse of the vast possibilities of Synthetic Biology.

Mouse with optic fibre. From Airan, et al., (2009)
In 2008 and 2009, other light-regulated systems were published: the fusion of LOV domains to the E. coli enzyme Dihydrofolate Reductase, in order to regulate its activity with light; light-regulated protein splicing with A. thaliana phytochromes; neural actvity regulation with light stimuli both in culture cells and living mice (this field of neurology research is known as optogenetics and, at the time, had already reported some results); the fusion of LOV-domains with histidine kinases, in order to regulate their activity and their signal transfer cascades in vivo; and finally, the regulation of cell motility by light in mammal cell lines.

But the work published in the paper  A Synthetic Genetic Edge Detection Program by Dr. Tabor and colleagues is worth a special mention, because they used two additional synthetic circuits: a Quorum Sensing communication module and a signal inverter. In this way, they coupled the light stimulus to the expression of a LacZ reporter for the detection of image edges, so they could reproduce the borders of an image in a Petri dish.

Multichromatic gene regulation. 
From Tabor, et al., (2011)

Finally, in 2011, Dr. J. J. Tabor, together with Dr. A. Levskaya and Dr. C. A. Voigt, reported an amazing advance for light regulation of genetic circuits. In Multichromatic Control of Gene Expression in Escherichia coli they describe how they were able to couple -in a single bacterial strain- a trancriptional response to two different light wavelenghts, using the chimeric EnvZ-Cph1 system (for red light sensing) and the two component CcaS-CcaR system from cyanobacteria (for green light sensing).

This recent advances in light regulation are very promising, but there are still some important challenges to be solved in order to achieve its application, like the signal transmission in a liquid medium. 


Nevertheless, research keeps going further and, with the creative impulse from the iGEM teams, we are for sure not far from the possibility of regulating gene expression from test tube to bioreactor scales using only light. 

Here are some interesting references:

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