Engineering Bacteria to Harvest Light

Commonly used lab bacteria called E. coli can be converted into light-harvesting organisms in a single genetic step, according to new research from MIT. The genetic enhancement allows microorganisms that normally derive their cellular energy from sugars to switch to a diet of sunlight. These findings could ultimately be used to genetically engineer bacteria that can more efficiently produce biofuels, drugs, and other chemicals.

Some bacteria, such as cyanobacteria, use photosynthesis to make sugars, just as plants do. But others have a newly discovered ability to harvest light through a different mechanism: using light-activated proteins known as proteorhodopsins, which are similar to proteins found in our retinas. When the protein is bound to a light-sensitive molecule called retinal and hit with light, it pumps positively charged protons across the cell membrane. That creates an electrical gradient that acts as a source of energy, much like the voltage, or electromotive force, supplied by batteries.

First discovered in marine organisms in 2000, scientists recently found that the genes for the proteorhodopsin system–essentially a genetic module that includes the genes that code for both the protein and the enzymes required to produce retinal–are frequently swapped among different microorganisms in the ocean. (While we usually think of genes being passed from parent to offspring, microorganisms can exchange bits of DNA laterally.)

Intrigued by the prospect that a single piece of DNA is really all an organism needs to harvest energy from light, the researchers inserted it into E. coli. They found that the microorganisms synthesized all the necessary components and assembled them in the cell membrane, using the system to generate energy. “All it takes to derive energy from sunlight is that bit of DNA,” saysEd Delong, professor of biological engineering at MIT and author of the study. The results were published last week in the Proceedings of the National Academy of Sciences.

The findings have implications for both marine ecology and for synthetic biology, an emerging field that aims to design and build new life forms that can perform useful functions. Giant genomic studies of the ocean have found that the rhodopsin system is surprisingly widespread. The fact that a single gene transfer can result in an entirely new functionality helps explain how this genetic module traveled so widely. In fact for microbes, this kind of module swapping may be the rule rather than the exception.”A new paradigm is emerging in microbiology: [microorganisms] are much more fluid than we thought,” says Ford Doolittle, Canada Research Chair in comparative genomics at DalhousieUniversity, in Nova Scotia.

These findings and other research on proteorhodopsins could provide biological engineers with a new tool to tinker with.A paper published last month by Jan Liphardt and colleagues at the University of California, Berkeley, showed that E. coli engineered to have a proteorhodopsin pump can easily switch between energy sources: when bacteria are starved of their regular energy supply, they use light energy to drive their flagellar motor, a rotating tail that bacteria use to swim. The more light there is, the faster the motor goes.

Rhodopsin pumps could eventually be engineered into the microbes commonly used to produce drugs and other chemicals. These bacterial factories sometimes run short on energy. “Using these light-driven proton pumps, bacteria can be energized by light to increase their yields of metabolites or pharmacologically active substances,” says John L. Spudich, professor of microbiology and molecular genetics at the University of Texas Medical School, in Houston. A cellular energy boost might come in particularly handy with the latest trend in bacterial production: engineering microbes to produce biofuels.

“It’s sort of like creating a hybrid car,” says MIT’s Delong. “Instead of supplementing gas with energy stored in a battery, cells can supplement their energy metabolism with light.”