Plants and bacteria often use light as a signal to drive a diverse array of cellular processes, ranging from transcriptional control to taxis. In order to accomplish this, they employ a class of proteins known as phytochromes. PixJ, a bacterial phytochrome that senses green and blue light from a photosynthetic bacterium found in hot springs, has a very interesting photoconversion mechanism. The photoconversion from the green absorbing to the blue absorbing state involves the formation of a covalent bond between the chromophore and a Cysteine sidechain. This process takes milliseconds to complete. In order to study this phenomenon with respect to both the protein motion and the chromophore, we are employing absorption and circular dichroism spectroscopies. The absorption spectroscopy will allow us to follow the electronic light absorption state of the chromophore during photoconversion, and the circular dichroism spectroscopy provides insight into the protein motion in the UV region and chromophore environment in the visible region. Using these three sources of data, we will piece together how the chromophore movement affects the protein structure and what role the protein structure has in the photoconversion process. We will also study the inverse reaction, from blue absorbing to green absorbing, which involves the rupture of the same covalent bond and takes a very long time, multiple seconds, to complete. With these tools at our disposal, we hope to acquire a holistic understanding of the photocycle of this phytochrome.

Insights into the mechanism of a green/blue phytochrome via absorption and circular dichroism spectroscopies

254th National Meeting (2017)

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