What we have shown in our project is only the tip of the iceberg. More possible applications of halorhodopin could benefit not only synthetic biology but also the human society. The following are some more advanced applications of halorhodopsin.
Halorhodopsin could facilitate light detection and light-coupled intercellular signal transduction. Light signal could be converted to intracellular chloride level signal, which can subsequently induce Pgad downstream genes. The target genes could be constructed to accelerate the synthesis pathway of quorum sensing signals, such as N-acyl homoserine lactones(AHL)1. As a result, one clone of bacteria could be designed as a light detector, amplifying and converting light signal to quorum signal which can cooperate with other clones of bacteria.
Moreover, halorhodopsin is not the only channel that couples light to ion transportation. There are other ion channels from retinylidene protein family sharing similar sequences and structures but pump other ions using light. An example is bacteriorhodopsin, which pumps protons out of the cells2 and channelrhodopsin non-specifically pumps cations into the cells3. Combining pH sensitive promoter4 with bacteriorhodopsin, osmolality sensitive promoters (OmpF, developed by iGEM08_NYMU-Taipei in 2008) and channelrhodopsin, the light coupled expression platform can be extended to a more accurate and more complex regulation system, making bacteria more programmable by computer.
Together with channelrhodopsin, halorhodopsin can enable seawater desalination with the help of solar energy. Seawater desalination has long been an attractive yet difficult task to implement efficiently. E. coli expressing halorhodopsin and channelrhodopsin could be one promising way to solve this problem. It is possible to drive E. coli to capture various ions (mainly sodium ion, potassium ion and chloride ion) in seawater and move these ions away by controlling the movement of E. coli. By this means seawater could be desalinated for drinking, irrigation or other daily usage.
In conclusion, halorhodopsin could fulfill the need of utilizing light as a signal or energy source. We believe that halorhodopsin would be one of the most promising tools of synthetic biology in the future.
1. Shapiro, J. a Thinking about bacterial populations as multicellular organisms. Annual review of microbiology 52,81-104(1998).
2. Hayashi, S., Tajkhorshid, E. & Schulten, K. Molecular dynamics simulation of bacteriorhodopsin’s photoisomerization using ab initio forces for the excited chromophore. Biophysical journal 85,1440-9(2003).
3. Nagel, G. et al. Channelrhodopsin-2, a directly light-gated cation-selective membrane channel. Proceedings of the National Academy of Sciences 100, 13940(2003).
4. San, K. et al. An Optimization Study of a pH-Inducible Promoter System for High-Level Recombinant Protein Production in Escherichiacoli. Annals of the New York Academy of Sciences 721,268–276(1994).
"Creativity is thinking up new things. Innovation is doing new things." - Theodore Levitt
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