PARENT SESSION
Posters P7B Evolution of photosynthesis. Abstracts (579-591)

Characterisation of higher plant Cytochrome c₆. Juergen Wastl^*,1, Fernando Molina-Heredia², Jose Navarro², Manuel Hervas², Derek Bendall¹, Miguel De la Rosa², Christopher Howe¹, ¹ Department of Biochemistry, Cambridge, United Kingdom² Instituto de Bioquimica Vegetal y Fotosintesis, Seville, Spain

ABSTRACT- Cytochrome c₆ is well known to transport electrons between the cytochrome bf complex and photosystem I in many cyanobacteria and algae, replacing plastocyanin in copper deficiency. It was widely believed that higher-plants lacked cytochrome c₆ until we reported a modified form for several higher plants from genomic evidence and EST sequencing (1). Alignment of higher plant cytochrome c₆ and algal/cyanobacterial cytochrome c₆ reveal a highly conserved 12 amino acid extension unique to the higher plant proteins. This implies an additional or different function in comparison with the cyanobacterial cytochromes. Gupta et al. (2) recently reported that heterologously expressed plant cytochrome c₆ could replace plastocyanin in reconstitution experiments in vitro, measuring oxygen evolution indirectly with inside-out thylakoids. However, we conclude from structural modelling and measured rates of reduction of Arabidopsis PSI particles that Arabidopsis cytochrome c₆ cannot play the same physiological role as plastocyanin or as algal cytochrome c₆ from Monoraphidium braunii (3). We found the UV/Vis absorption spectrum of reduced Arabidopsis cytochrome c₆ expressed in E.coli to be similar to that of Monoraphidium cytochrome c₆, except that the absorption bands are shifted slightly towards the red. The peak is at 554.5 and 552.5 nm for plant and algal cytochromes, respectively. Biophysical properties of cytochrome c₆ and a mutant form lacking the higher plant specific 12 amino acid loop are compared and the structural and functional importance is discussed. 1) Wastl, J., Bendall, D.S. & Howe, C.J. Trends Plant Sci. 7, 244-245 (2002). 2) Gupta, R., He, Z. & Luan, S. Nature 417, 567-571 (2002). 3) Molina-Heredia, F.P.,Wastl, J., Navarro, J.A., Bendall, D.S, Hervas, M., Howe, C.J. & De la Rosa, M.A. Nature 424, 33-34 (2003)

KEY WORDS: photosystem I, cytochrome c₆ , electron transfer