12-03 :: March/April 2012

nanotimes News in Brief

At the heart of the biosensor is the ribose-binding protein, which, as the name suggests, attaches to the sugar ribose. Each ribose-binding protein is then flanked by two other proteins – one that glows blue and another that glows yellow. This three-protein complex attaches to the silica shell while the diatom grows.

In the absence of ribose, the two fluorescent proteins sit close to one another. They‘re close enough that the energy in the blue protein‘s fluorescence is easily handed off, or transferred, to the neighboring yellow protein. This process, called fluorescence resonance energy transfer, or FRET, is akin to the blue protein shining a flashlight at the yellow protein, which then glows yellow.

But when ribose binds to the diatom, the ribose-bin- ding protein changes its shape. This moves the blue and yellow fluorescent proteins apart in the process, and the amount of light energy that the blue protein shines on the yellow protein declines. This causes the biosensor to display more blue light.

Regardless of whether or not ribose is bound to the diatom‘s biosensor, the biosensor always emits some blue or yellow glow when it‘s exposed to energy under a microscope. But the key difference is how much of each kind of light is displayed.

The PNNL team distinguished between light from the two proteins with a fluorescence microscope that was equipped with a photon sensor. The sen- sor allowed them to measure the intensities of the unique wavelengths of light given off by each of the fluorescent proteins. By calculating the ratio of the two wavelengths, they could determine if the diatom

PNNL researchers genetically engineered this microscopic marine diatom to become a biosensor for the sugar ribose. From left to right: The engineered diatom without fluo- rescence; the same diatom exhibiting blue fluorescence; and, when no ribose is present, the diatom generates a bright yellow fluorescence via fluorescence resonance energy transfer. © Amnis Corp. of Seattle, Wash., used its ImageStream imaging flow cytometer to take these images.

45

biosensor was exposed to ribose, and how much of ribose was present. The team also succeeded in making the biosensor work with the shell alone, after it was removed from the living diatom. Removing the living diatom provides researchers greater flexibility in how and where the silica biosensor can be used. The Office of Naval Research, which funded the research, believes biosensors based on modifying a diatom‘s silica shell may prove useful for detecting threats such as explosives in the marine environment.

Kathryn E. Marshall, Errol W. Robinson, Shawna M. Hen- gel, Ljiljana Paša-Tolić, Guritno Roesijadi: FRET Imaging of Diatoms Expressing a Biosilica-Localized Ribose Sensor, In: PLoS ONE, 7(3), e33771, March 21, 2012, DOI:10.1371/ journal.pone.0033771: http://dx.doi.org/10.1371/journal.pone.0033771