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

Photosystem I lacking the PSI-G subunit is more active and less stable. Agnieszka Zygadlo^*,, Poul Erik Jensen, Henrik Vibe Scheller,

ABSTRACT- PSI-G is an 11 kDa subunit of photosystem I (PSI) in photosynthetic eukaryotes. Arabidopsis plants devoid of PSI-G have increased activity of NADP+ photoreduction and decreased PSI content but otherwise no obvious phenotype. Investigations of Arabidopsis plants devoid of PSI-G revealed interesting information about the function of PSI-G. The kinetic parameters of the reaction between PSI and plastocyanin were determined in the absence of PSI-G and compared to wild type. PSI-G clearly is involved in the interaction of plastocyanin with PSI since the dissociation constant (Kd) is three times lower in the absence of PSI-G compared to wild type. On the physiological level plants devoid of PSI-G have a more reduced plastoquinol pool which was correlated with an increased phosphorylation of thylakoid proteins such as the light harvesting complex II. Plants devoid of PSI-G were also found to be more photoinhibited than the wild type both at low temperature (PSI photoinhibition) and after high light treatment (PSII photoinhibition). Thus, despite being more active in the absence of PSI-G it seems that PSI-G also is involved in PSI stability. This was confirmed by measuring PSI activity as NADP+ photoreduction activity after illumination of a thylakoid suspension which clearly showed a faster decrease in PSI activity in the thylakoids lacking PSI-G. Light response dependence of the P700 redox state showed that in the absence of PSI-G, P700 is more reduced at low light intensities. We conclude that PSI-G is directly involved in the binding dynamics of plastocyanin to PSI and that PSI-G also is important for stability of the PSI complex.

KEY WORDS: plastocyanin, photosystem I, dissociation constant Kd