Light controlled photosynthesis in the metabolically versatile bacterium Rps palustris

The bacterium Rps. palustris is a metabolically versatile photosynthetic organism that can assimilate a variety of carbon sources including the greenhouse gas CO2 and the important plant polymer lignin. It is therefore important for understanding how carbon is recycled in the environment. It is also of interest as a source of biofuels. The bacterium has light-harvesting complexes that capture solar energy and which are designed to be optimal in the near IR between 800-880 nm. The bacteria can experience low-light conditions because the water they live in attenuates significantly in the near IR and because of light absorption by other photosynthetic organisms. It has an unusual light-harvesting complex (LH4), that is expressed under low-light conditions, with unique spectroscopic properties. The organism is also unusual in containing 6 bacteriophytochromes which are two-component systems that are driven by light and control gene expression. Three of the Bphs have been shown to be involved in controlling LH4 expression while Bph3 is responsible for controlling a large cluster of genes responsible for photosynthesis. These include LH1/RC peptides and several enzymes responsible for the biosynthesis of chromaphores. Bphs contain a biliverdin chromaphore that undergoes a Z to E isomerisation on red light illumination producing the Pfr form which reverts to the Pr state on far-red light illumination. The movement is transduced through the PHY domain which changes the efficiency of autophosphorylation in the histidine kinase domain and also affects the rate of release of the phosphate to an Asp residue of a response regulator. Bph3 has a very different C-terminal domain which does not involve phosphorylation and is believed to directly compete with the DNA for binding of the repressor PpsR2. Three spectroscopically active Bphs and a reduced protein containing only the chromaphore binding domain (CBD) have been expressed and characterised. A low resolution structure of LH4 has been determined and the complex has been characterised by near IR absorption, CD and single molecule spectroscopy techniques. High resolution structures of Bphs and LH4 are required to understand the details of biological action. In particular how light is being used for signalling and at the same time as a source of energy to optimally photosynthesise in low-light conditions. Light induced shape changes will be investigated by Small Angle X-ray Scattering and the dynamics of light-transduction and energy transfer will be characterised by time resolved spectroscopy and interpreted in the light of the structure. One of the objectives of using time resolved spectroscopy techniques is to understand the requirements of ultra fast IR, Raman and 2D-Spectroscopy methods and the potential of these methods on fourth generation light sources such as 4GLS.