PARENT SESSION
Posters P8B Supermolecular organization of the photosynthetic apparatus. Abstracts (592-611)

The PSI-O subunit is involved in balancing the excitation pressure between the two photosystems. Poul Erik Jensen^*,1, Anna Haldrup¹, Suping Zhang¹, Henrik Scheller¹, ¹ Plant Biochemistry Laboratory, Frederiksberg C, Denamrk

ABSTRACT- PSI-O is a subunit of photosystem I in eukaryotes. The function of PSI-O was characterized in Arabidopsis plants using RNA interference (RNAi). Several transformants with the psaO-RNAi construct were generated and a high proportion of the plants contained virtually no residual PSI-O. Plants lacking PSI-O have a 50% reduction in state transitions indicating a role for PSI-O in the balancing of excitation energy between the two photosystems. PSI-H and PSI-L have previously been shown to be involved in state transitions and immunoblot analysis revealed that plants devoid of PSI-L or PSI-H also have 80-90% reduction in the abundance of PSI-O. In contrast, downregulation of PSI-O has no negative effect on the content of PSI-H and PSI-L. The interaction between PSI-O and the PSI-L was confirmed by chemical cross-linking. Digitonin-PSI was purified to obtain an LHCII-PSI complex; and two to three times more LHCII was associated with PSI in the wild type in State 2 than in State 1. PSI from plants lacking PSI-H, PSI-L or PSI-O contained only about 30% of Lhcb1 compared with the wild type. Cross-linking prior to sucrose gradient purification resulted in copurification of LHCII in the wild type, but not with PSI from PSI-H, PSI-L and PSI-O mutants. A model of PSI is proposed in which PSI-L as the most ancient subunit is closest to the reaction center and PSI-O is positioned close to PSI-L on the PSI-H/L/I side of the PSI complex. PSI-H, PSI-L, PSI-O and possibly PSI-I are all involved in forming a domain in PSI which is involved in interaction with LHCII. Some of the subunits interact directly with LHCII, and others may be required for the stability or assembly of the domain.

KEY WORDS: RNA interference, photosystem I, State 1 - State 2 transitions