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Phycobilisomes Supply Excitations to Both Photosystems in a Megacomplex in Cyanobacteria

Science  29 Nov 2013:
Vol. 342, Issue 6162, pp. 1104-1107
DOI: 10.1126/science.1242321

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Complexing Photosynthesis

Photosynthesis operates through a series of protein complexes that harvest sunlight and turn it into chemical energy. The separate complexes—including photosystems I and II, phycobilisome antennae, and reaction centers—are understood for a number of photosynthetic organisms; however, the large-scale organization and interactions between them are less clear. Using protein cross-linking, Liu et al. (p. 1104) demonstrate how the individual components are organized when present as a megacomplex in the cyanobacterium Synechocystis PCC 6803. Time-resolved fluorescence spectroscopy indicated that the phycobilisomes transfer energy to both photosystems, which is consistent with their molecular arrangement.

Abstract

In photosynthetic organisms, photons are captured by light-harvesting antenna complexes, and energy is transferred to reaction centers where photochemical reactions take place. We describe here the isolation and characterization of a fully functional megacomplex composed of a phycobilisome antenna complex and photosystems I and II from the cyanobacterium Synechocystis PCC 6803. A combination of in vivo protein cross-linking, mass spectrometry, and time-resolved spectroscopy indicates that the megacomplex is organized to facilitate energy transfer but not intercomplex electron transfer, which requires diffusible intermediates and the cytochrome b6f complex. The organization provides a basis for understanding how phycobilisomes transfer excitation energy to reaction centers and how the energy balance of two photosystems is achieved, allowing the organism to adapt to varying ecophysiological conditions.

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