Multiple LHCII antennae can transfer energy efficiently to a single Photosystem I

Photosystems I and II (PSI and PSII) work in series to drive oxygenic photosynthesis. The two photosystems have different absorption spectra, therefore changes in light quality can lead to imbalanced excitation of the photosystems and a loss in photosynthetic efficiency. In a short-term adaptation response termed state transitions, excitation energy is directed to the light-limited photosystem. In higher plants a special pool of LHCII antennae, which can be associated with either PSI or PSII, participates in these state transitions. It is known that one LHCII antenna can associate with the PsaH site of PSI. However, membrane fractions were recently isolated in which multiple LHCII antennae appear to transfer energy to PSI. We have used time-resolved fluorescence-streak camera measurements to investigate the energy transfer rates and efficiency in these membrane fractions. Our data show that energy transfer from LHCII to PSI is relatively slow. Nevertheless, the trapping efficiency in supercomplexes of PSI with ~ 2.4 LHCIIs attached is 94%. The absorption cross section of PSI can thus be increased with ~ 65% without having significant loss in quantum efficiency. Comparison of the fluorescence dynamics of PSI-LHCII complexes, isolated in a detergent or located in their native membrane environment, indicates that the environment influences the excitation energy transfer rates in these complexes. This demonstrates the importance of studying membrane protein complexes in their natural environment.