PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Structural analysis of PSI-LHC supercomplexes of state I and state I forms from Chlamydomonas reinhardtii. Rajagopal Subramanyam^{*,1, 3}, Ingo Grotjohann^{2, 3}, Jean-David Rochaix⁴, Petra Fromme^{2, 3}, Andrew Webber^{1, 3}, ¹ School of Life Sciences, Arizona State University, Tempe, Arizona, USA³ Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona, USA² Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, USA⁴ Departments of Molecular Biology and Plant Biology, University of Geneva,12111 Geneva 4, Switzerland

ABSTRACT- A photosystem I (PSI) supercomplex that includes the chlorophyll a/b light-harvesting complexes (LHC) been isolated using sucrose gradient centrifugation from wild type and the stt7 mutant (a mutant which can not perform state transition from State I to State II) of Chlamydomonas reinhardtii acclimated to Sate I or State II conditions. We have compared the polypeptide composition of the different supercomplexes using SDS-PAGE and are investigating structural differences by electron microscopy of negatively stained samples of the PSI-LHC supercomplexes isolated under State I and State II conditions. After sucrose gradient centrifugation, two PSI-LHC green bands were resolved. In wild type, the PSI-LHC supercomplexes from cells in the State I condition accumulated less of the lower band and more of the upper band, whereas in state II conditions the abundance of the lower band increased. The chlorophyll a/b ratio was also different in the two bands. In the stt7 mutant (locked in the State I), the majority of the PSI-LHC supercomplexes were in the upper band. SDS-PAGE analysis of the two forms of PSI-LHC supercomplexes shows significant differences in the subunit composition. Negatively stained electron micrographs show that in the State I condition, the dimension of the particles are 150 in both wild type and the stt7 mutant, whereas particles in the state II conditions are 250 . The electron micrograph images show that the PSI-LHC supercomplexes from Chlamydomonas are different from the structure of the pea PSI-LHCI supercomplexes published recently based on the crystal structure.

KEY WORDS: PSI-LHC supercomplexes, chlamydomonas, elctron micrographs, State I and State II