Dissecting the function of the LH2 pucBA multigene family in Rhodospeudomonas palustris.

Lead Research Organisation: University of Glasgow
Department Name: College of Medical, Veterinary, Life Sci

Abstract

Purple photosynthetic bacteria, such as Rhodopsuedomonas palustris, have proved to be excellent model organisms for use in trying to understand the detailed molecular mechanisms involved in the early reactions of photosynthesis. Photosynthesis begins with the absorption of solar energy by pigment-protein complexes called light harvesting complexes. Rps. palustris contains a multigene faimly that encodes light harvesting complexes that have different absorption spectra. These different complexes allow the bacterium to grow over a wide range of incident light intensities and are therefore important for allowing them to be competitive in the wild. This project aims to understand how the different members of this multigene family produce light harvesting complexes wth different absorption spectra and how this enables them to harvest light energy efficiently at different incident light intensities. This information is not only fundamental in trying to understand a basic piece of biology but also for research into bioenergy. One way to use solar energy more efficiently is to devise ways of carrying out artificial photosynthesis. Producing such systems requires the design of the initial solar energy collectors ie. the light harvesting complexes. The results of this project will reveal how to control where light harvesting pigments absorb and so aid the design of such artificial solar energy collectors.

Technical Summary

Many species of purple photosynthetic bacterial have a multigene family that encode the LH2 apaoproteins. Rhodopsuedomonas palstris is such a species and is able to synthesise LH2 complexes where individual LH2 rings have a heterogeous apoprotein composition. The different rings hsave different near infared absotption spectra. This research project sets out a program of experiments designed to determine how the different arrangement of the differebnt apoproteins in the individual LH2 rings controls where the ring absorbs light energy. A set of deletion strains, where specific LH2 apoprotein genes have been deleted, will allow LH2 complexes with defined ,restricted apoprotein compositions to be synthesised. The structure and spectroscopic properties of these LH2 complexes will be correlated with their specific apoprotein composition. By a combination of these studies and mathematical modelling it will be possible to fully describe how their precise structure controls where they absorb in the near infared region of the spectrum. Finally energy transfer measurements on membranes prepared from the diferent deletion strains will be used to determine how the different spectroscopic forms effect overall light harvesting performance, and thereby to understand the selective advantage to ability to synthesise the different types of LH2 confers.

Planned Impact

The main beneficiaries of this proposed research project will be the academic community interested in a basic understanding of the importance of light harvesting in photosynthesis. This understanding is important for anybody who wishes to try to produce systems capable of performing artificial photosynthesis at higher efficiencies than can currently be achieved by the natural process. Ultimately this information could be of importance to any companies wishing to manufacture systems capable of producing solar fuels. These impacts will be realised by us disseminating all the results of this project in open access publications, conference proceedings and direct interactions with industry through Glasgow University's annual industry days. The concept of solar fuels and the land area this will require involves imoportant communication with the general public and policy makers. RJC has strong interactions with local school teachers and to the media via the BBC. He will use these contacts to publicise these issues and to promote as much public debate upon them as possible.
The Post. Doc. on this grant, Dr. Sarah Henry , will be fully involved in these interations with both industry and the general public. As a result of this and the general training in a wide range of different experimental methods she will be a excellent researcher to move over to work in industry at the end of this grant.
 
Description That our deletion mutants do allows us to understand the role of each individual puc gene.
Exploitation Route We are providing our different LH2 samples to many groups world wide.
Sectors Education,Energy
URL http://pubs.rsc.org/en/content/articlepdf/2018/cp/c7cp06139k
 
Description Quantum Coherent Energy Transfer over Varying Pathways in Single Light-Harvesting Complexes 
Organisation University of Bayreuth
Country Germany 
Sector Academic/University 
PI Contribution As a result of the primary collaboratation with Professor Koehler, this extra collaboration was arranged. The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction centre. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.
Start Year 2012
 
Description Single-molecule spectroscopy reveals photosynthetic LH2 complexes switch between emissive states 
Organisation University of Bayreuth
Country Germany 
Sector Academic/University 
PI Contribution As a result of the primary collaboratation with Professor Koehler, this extra collaboration was arranged. Photosynthetic organisms flourish under low light intensities by converting photoenergy to chemical energy with near unity quantum efficiency and under high light intensities by safely dissipating excess photoenergy and deleterious photoproducts. The molecular mechanisms balancing these two functions remain incompletely described. One critical barrier to characterizing the mechanisms responsible for these processes is that they occur within proteins whose excited-state properties vary drastically among individual proteins and even within a single protein over time. In ensemble measurements, these excited-state properties appear only as the average value. To overcome this averaging, we investigate the purple bacterial antenna protein light harvesting complex 2 (LH2) from Rhodopseudomonas acidophila at the single-protein level. We use a room-temperature, single-molecule technique, the anti-Brownian electrokinetic trap, to study LH2 in a solution-phase (nonperturbative) environment. By performing simultaneous meas-urements of fluorescence intensity, lifetime, and spectra of single LH2 complexes, we identify three distinct states and observe transitions occurring among them on a timescale of seconds. Our results reveal that LH2 complexes undergo photoactivated switching to a quenched state, likely by a conformational change, and thermally revert to the ground state. This is a previously unobserved, reversible quenching pathway, and is one mechanism through which photosynthetic organisms can adapt to changes in light intensities.
Start Year 2012