Introduction to the Results
Proteorhodopsin
Proteorhodpsin is a light activated proton pump that exploits the conformational change of all trans-retinal to all cis-retinal. The different absorption properties are due to a single amino acid, at position 105 in the retinal binding pocket. The presence of a highly conserved Gln at position 105 in BBa_K1604010 indicates that it belongs to the blue absorbing family. [3]
We tested if light activation with a white light bulb (160W) containing the blue wavelength, activates proteorhodopsin, thus making the bacteria survive better anaerobically.
Anaerobiosis was achieved using sealed glass bottles with a rubber septum. We got from the local pharmacy 12 sterile bottles of physiological solution. After removing the liquid, washing them and autoclaving them, the bottles were ready to host our bacteria!
After five hours of induction in the dark (i.e. the samples were wrapped in aluminum foils) the cultures were split in the anaerobic chamber in light and dark conditions. The cultures were placed in the thermoshaker that was illuminated from the outside. Half of the cultures were kept in the dark and the other half were exposed to the light.
The OD600 was constantly monitored because E. coli’s growth is slowed down in stressful conditions such as the lack of oxygen.
The bacteria expressing proteorhodopsin have an increased lifetime when compared to a negative control with araC-pBAD (BBa_K731201). However we did not observe significant changes between light and dark with this test. The explanations could be several. Most likely we were not exciting properly the system. However it seems that there is a basal functionality even in the absence of light, probably due to activation of the proton pump independently from light exposure.
While we decided to explore different light sources, we built a solar mimicking apparatus, that would allow us to directly illuminate the samples without the glass of the thermoshaker.
To sum up...
Part improvement
We successfully improved BBa_K773002 and now it works! Our PR was expressed in E. coli NEB10β cells and functionally characterized.
More ATP, better survival
E. coli equipped with proteorhodopsin survive better under anaerobic condition by producing higher levels of ATP
Towards the pMFC
Proteorhodopsin-engineered bacteria are happy to stay under the sun in our Microbial Fuel Cell.
Check out our Solar pMFC results.
PncB: nicotinic acid phosphorbosyl-transferase
Our device is controlled by an inducible arabinose promoter built by the Unitn iGEM team in 2012. PncB was extracted by E. coli genome, the illegal site PstI was removed, and it was placed in pSB1C3 (BBa_K1604030). Subsequently it was placed under the araC-pBAD promoter (BBa_K1604030).
PncB enhances NAD production by ~2.5 fold
Our goal was to demonstrate that pncB increased intracellular levels of NAD and thus NADH. We quantified the levels of NAD by a colorimetric test that measures the levels of NAD indirectly by quantifying the concentration of NAD total (NAD + NADH) and NADH only. To make precise quantitation a standard curve with NADH was built. The test provides the ratio of NAD/NADH
NADtotal = Amount of total NAD (NAD+NADH) in unknown sample (pmole) from standard curve.
NADH = Amount of NADH in unknown sample (pmole) from standard curve.
BBa_K1604031 does increase NAD levels by 126% (2.5 fold) and NADH levels by 44% (1.4 fold) when expressed in NEB10β. Although we did see an enhancement in NAD levels, this did not correlate to a proportional boost in NADH levels. We plan in the future to add a NAD reducing enzyme and to give a medium able to enhance the cell metabolism to further increase NADH intracellular levels.