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Photosynthesis: How a Cyanobacterium Makes Far-Red Light Mean “Go”

When cyanobacteria live in low-light conditions, such as beneath a pond surface or under a plant canopy on a forest floor, some are able to switch from using the visible light that is most conducive to their growth and photosynthetic activities to harvesting the weaker, far-red sunlight that filters down to them. The current study provides the structural basis for the ability of such cyanobacteria to use far-red light for oxygen-evolving photosynthesis. Credit: Shireen Dooling, Biodesign Institute at Arizona State University

Researchers identify the locations of structural changes in photosystems I and II that allow growth in far-red light.

When grown under normal, “white” light conditions—that is, visible light, which ranges from violet light with a wavelength of about 400 nm to red at 700 nm—cyanobacteria harvest that light using mainly chlorophyll a, which absorbs light with wavelengths up to a maximum of about 700 nm. When grown in far-red light (up to about 800 nm), some terrestrial cyanobacteria convert a portion of that chlorophyll a into chlorophylls d and f, which absorb longer wavelengths of light. These alternative forms of chlorophyll give such organisms the ability to harvest far-red light and use it efficiently for photosynthesis, which allows those cyanobacteria to thrive in low- or filtered-light environments, such as occurs under plants or trees.

“We knew from isolating and characterizing the complexes that photosystem I contains 7 to 8 chlorophyll f molecules, and that photosystem II contains one chlorophyll d molecule and 4 to 5 chlorophyll f molecules, along with about 90 percent of the original chlorophyll a, so we wanted to know where those changes occurred in the complexes,” said Bryant. “One way to figure that out is to determine the structure of the complexes, but because they are so large and complex—and the chemical differences are so minor—it was extremely challenging.”

The photosystem I and II complexes are very difficult to crystallize—because they are very large, membrane-bound complexes—so X-ray crystallography, a standard laboratory method for determining the three-dimensional structures of molecules, was not likely to work. The researchers then turned to cryo-EM, but the tiny differences between the forms of chlorophyll molecules stretched the limits of cryo-EM resolution to detect. The chlorophylls differ at only a few atoms of similar mass.

“My collaborator, Chris Gisriel, who is a postdoctoral fellow in Gary Brudvig’s lab at Yale, was fortunate to achieve a very high-resolution structure for the photosystem II complex—2.25 angstrom (Å)—allowing him to visualize the differences in some of the chlorophylls directly,” said Bryant. “The extent of the difference between chlorophyll a and f is that two hydrogen atoms are replaced by an oxygen atom

An atom is the smallest component of an element. It is made up of protons and neutrons within the nucleus, and electrons circling the nucleus.

” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>atom in a molecule with the composition of C55H72MgN4O5. In a complex like photosystem I that contains nearly 100 pigment molecules and 11 protein subunits or photosystem II with 35 chlorophylls and 20 protein subunits, these small changes are like looking for a few needles two very large haystacks. Because these chlorophylls confer the special properties that allow far-red light utilization, it is very important to understand exactly how these molecules are arranged.”

Most of the time, the oxygen atoms are tied up in hydrogen bonds, so the researchers can look for hydrogen-bond donors that are close to the right places in the chlorophyll molecules. By applying this method and others to the structures determined using cryo-EM, they were able to identify the locations of chlorophyll f molecules in the two photosystem complexes and the position of the single chlorophyll d molecule in photosystem II as well.

“Identifying the structural basis for how this far-red light-absorption occurs in nature is an important step forward,” said Gisriel, first author of both studies. “The identification of the precise locations in the photosystem I and II complexes where the alternate forms of chlorophyll are incorporated could open up the doors for exciting future applications. For example, crops could potentially be engineered to harvest light beyond the visible spectrum. In addition, two crops could potentially be grown together, with shorter crops, using the filtered far-red light from their shaded locations beneath taller crops. Alternatively, plants could be grown closer together because of better light capture in the leaves beneath the canopy.”

In addition to Bryant and Gisriel, the research team for the first paper, titled “Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides insights into far-red light photoacclimation,” includes, David A. Flesher, Gaozhong Shen, Jimin Wang, Ming-Yang Ho, and Gary W. Brudvig. Funding was provided by the U.S. National Science Foundation and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.

The research team for the second paper, titled “Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f,” includes Gaozhong Shen, Ming-Yang Ho, Vasily Kurashov, David A. Flesher, Jimin Wang, William H. Armstrong, John H. Golbeck, M.R. Gunner, David J. Vinyard, Richard J. Debus, and Gary W. Brudvig. The research was supported by the U.S. National Science Foundation, the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, the U.S. Department of Energy, Division of Chemical Sciences, Geosciences, and Biosciences, Photosynthesis Systems, and the National Institute of General Medical Sciences of the U.S. National Institutes of Health.

References:

“Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides insights into far-red light photoacclimation” by Christopher J. Gisriel, David A. Flesher, Gaozhong Shen, Jimin Wang, Ming-Yang Ho, Gary W. Brudvig and Donald A. Bryant, January 2022, Journal of Biological Chemistry.
DOI: 10.1016/j.jbc.2021.101408

“Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f” by Christopher J. Gisriel, Gaozhong Shen, Ming-Yang Ho, Vasily Kurashov, David A. Flesher, Jimin Wang, William H. Armstrong, John H. Golbeck, Marilyn R. Gunner, David J. Vinyard, Richard J. Debus, Gary W. Brudvig and Donald A. Bryant, January 2022, Journal of Biological Chemistry.
DOI: 10.1016/j.jbc.2021.101424

Source: SciTechDaily