Exploring the Roots of Photosynthesis in a Soil-Dwelling Bacterium

structure H. modesticaldum
Overall structure of the H. modesticaldum photosynthetic reaction center. The transmembrane helices are shown in gold and red. Light-harvesting “antenna” molecules are colored green, and the electron-transfer chain is colored blue. [From Gisriel, C., et al. “Structure of a Symmetric Photosynthetic Reaction Center–Photosystem,” Science 357(6355), 1021–1025 (2017). DOI:10.1126/science.aan5611. Reprinted with permission from AAAS.]
The simplest known bacterium able to drive photosynthesis is found in muddy soils near hot springs. Heliobacterium modesticaldum is a sun-loving, soil-dwelling, thermophilic bacterium that photosynthesizes near-infrared light, unlike plants, which use different parts of visible light. The photosynthesis reaction centers (RCs) of H. modesticaldum are thought to resemble the earliest common ancestor of all photosynthesis complexes, which evolved around three billion years ago. Thus, a clear, detailed picture of the H. modesticaldum RC would provide valuable insight into the early evolution of photosynthesis. However, successfully purifying an RC protein and growing the crystals needed for X-ray crystallography can be a lengthy, difficult process.

Researchers successfully capped 7 years of work by obtaining a high-resolution (2.2 Å) structure of the membrane protein at the heart of the photosynthetic RC of H. modesticaldum. The structure revealed details such as the interactions between light-harvesting “antenna” molecules and the electron-transfer chain, where the initial steps required to convert photon energy into chemical energy occurs. With this data, the research team was able to make comparisons to other types of RCs, gaining new perspectives on the early evolution of photosynthesis and how nature optimized light-driven energy collection. This work will help unlock the secrets of photosynthesis, possibly leading to the development of cleaner, solar-based renewable energy.

Gisriel, C., et al. “Structure of a Symmetric Photosynthetic Reaction Center–Photosystem.” Science 357(6355), 1021–1025 (2017). [DOI:10.1126/science.aan5611].

Instruments and Facilities: Beamline 8.2.1 at Advanced Light Source at Lawrence Berkeley National Laboratory; Advanced Photon Source at Argonne National Laboratory.