Award details

Assembly and evolution of a photosynthetic antenna

Reference	BB/W008076/1
Principal Investigator / Supervisor	Dr Daniel Canniffe
Co-Investigators / Co-Supervisors
Institution	University of Liverpool
Department	Biochemistry & Systems Biology
Funding type	Research
Value (£)	474,983
Status	Current
Type	Research Grant
Start date	01/09/2022
End date	31/08/2025
Duration	36 months

Abstract

The majority of purple phototrophic bacteria use BChl a for phototrophy, but Rhodospirillum (Rsp.) rubrum synthesises a form of this pigment with an unreduced, inflexible alcohol moiety. Unusually, Rsp. rubrum also lacks the common peripheral light-harvesting antenna (LH2), relying solely on the reaction centre-light harvesting 1 complex (RC-LH1) for energy capture and conversion. A link between these two characteristics has been determined; I have demonstrated that LH2 can assemble in Rsp. rubrum only when reduced BChl a is produced, defining the factors permitting assembly of this complex. Rsp. rubrum RC-LH1 is housed in spherical membrane vesicles, chromatophores, with a larger diameter than those found other purple bacteria, with LH2 implicated in imparting extreme curvature on these chromatophores. Classical mutagenesis and accelerated evolution under LH2-specific wavelengths will be used to generate a foreign LH2 that displays enhanced coordination with the native RC-LH1, to create an efficient system able to harvest unused wavelengths in the 780-850 nm range. Assembly of LH2 in Rsp. rubrum may increase membrane curvature, resulting in smaller but more numerous chromatophores, with the potential to increase membrane surface area, and therefore space in which to house a larger number of antennas, increasing light-harvesting capacity. The effect of foreign LH2 assembly, and its optimisation via laboratory evolution, will be analysed by growth experiments. Increased coordination with RC-LH1 in the system will be monitored by AFM, and the effect of LH2 incorporation on membrane morphology and area will be studied using cryo-electron tomography. Information derived from this project will be used to design light-harvesting complexes targeted to specific regions of the solar spectrum, a key component of proposals aimed at the radical redesign of photosynthesis, and can be used to determine and regulate the ultrastructure of the energy-transducing membrane

Summary

Photosynthesis is the source of all the food we eat, and almost all of the energy we use. This process uses sunlight to remove carbon dioxide from the atmosphere and convert it into carbohydrates that feed the planet. Sunlight is captured by chlorophyll pigments that are arranged and held in place by proteins; these pigment-protein arrangements are known as antenna complexes. Antennas collect the light energy and funnel it towards specialised 'reaction centres', where the energy is converted to a form that can be used by the cell. Plants and cyanobacteria (blue-green algae) use chlorophyll (Chl) pigments to capture visible light (400-700 nm) to perform 'oxygenic' photosynthesis, releasing the oxygen that supports respiration. Additionally, a diverse assortment of bacteria are also capable of using light outside this range (>700 nm), which we cannot see but feel as heat, to perform 'anoxygenic' photosynthesis. This mode of photosynthesis relies on the bacteriochlorophyll (BChl) pigments, rather than Chls. The majority of anoxygenic photosynthesisers use BChl a to harvest light between 750-900 nm, although Rhodospirillum rubrum is a well-studied example that unusually cannot harvest light effectively up to 850 nm because it lacks the common antenna complex. This project aims to transfer the antenna of another photosynthetic bacterium to Rhodospirillum rubrum, to allow the new, hybrid organism to capture light it was not previously able to. Further modifications to the new bacterium will be made by targeted alterations to the genome, and mutations will also be naturally acquired by growing the organism under light that can only be absorbed by the new antenna complex, a process that mirrors natural evolution, but that can be speeded-up in the laboratory. Achieving these aims will reveal how to assemble and regulate the production of pigment-protein complexes in other simple bacteria, with the long-term goal of putting boosted light-capturing ability to use to tacklesome of humanity's impending fuel and food supply challenges in a sustainable manner. This could also have a positive effect on climate change; increased removal of CO2 greenhouse gas, and its conversion into sugars, could slow the warming of the planet, and mitigate the damage to the environment.

Committee	Research Committee B (Plants, microbes, food & sustainability)
Research Topics	Microbiology, Structural Biology, Synthetic Biology
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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