A natural light switch
Scientists have discovered and mapped a light-sensing protein that uses vitamin B12
The result, derived from studying proteins from the bacterium Thermus thermophilus, involves at least two findings of broad interest. First, it expands our knowledge of the biological role of vitamin B12, which was already understood to help convert fat into energy, and to be involved in brain formation, but has now been identified as a key part of photoreceptor proteins.
Second, the research describes a new mode of gene regulation, in which the light-sensing proteins play a key role. In so doing, the scientists observe, the bacteria have repurposed existing protein structures that use vitamin B12, and put them to work in new ways.
"Nature borrowed not just the vitamin, but really the whole enzyme unit, and modified it ... and made it a light sensor," says Catherine Drennan, a professor of chemistry and biology at MIT.
The work describes the photoreceptors in three different states: in the dark, bound to DNA, and after being exposed to light.
"It's wonderful that we've been able to get all the series of structures, to understand how it works at each stage," Drennan says.
The researchers used a combination of X-ray crystallography techniques and in-vitro analysis to study the bacteria. Drennan, who has studied enzymes that employ vitamin B12 since she was a graduate student, emphasizes that key elements of the research were performed by all the co-authors.
Jost performed crystallography to establish the shapes of the structures, while the Spanish researchers, Drennan notes, "did all of the control experiments to show that we were really thinking about this right," among other things.
By studying the structures of the photoreceptor proteins in their three states, the scientists developed a more thorough understanding of the structures, and their functions, than they would have by viewing the proteins in just one state.
Microbes, like many other organisms, benefit from knowing whether they are in light or darkness. The photoreceptors bind to the DNA in the dark, and prevent activity pertaining to the genes of Thermus thermophilus. When light hits the microbes, the photoreceptor structures cleave and "fall apart," as Drennan puts it, and the bacteria start producing carotenoids, which protect the organisms from negative effects of sunlight, such as DNA damage.
The research also shows that the exact manner in which the photoreceptors bind to the DNA is novel. The structures contain tetramers, four subunits of the protein, of which exactly three are bound to the genetic material -- something Drennan says surprised her.
"That's the best part about science," Drennan says. "You see something novel, then you think it's not really going to be that novel, but you do the experiments [and it is]."
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