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

The Arabidopsis chloroplastic NifU-like protein AtCnfU can act as an iron-sulfur cluster scaffold protein and is required for biogenesis of ferredoxin and Photosystem I. Toshiki Yabe^*,1, Kozo Morimoto¹, Shingo Kikuchi¹, Kazuaki Nishio², Ichiro Terashima³, Masato Nakai¹, ¹ Institute for Protein Research, Osaka University, Suita, Osaka, Japan² The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan³ Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan

ABSTRACT- The biosynthesis of iron-sulfur clusters is a highly regulated process involving several proteins. Among them, so-called scaffold proteins play pivotal roles in both the assembly and delivery of iron-sulfur clusters. Here, we report the identification of two chloroplast-localized NifU-like proteins, AtCnfU-V and AtCnfU-IVb from Arabidopsis thaliana with high sequence similarity to a cyanobacterial NifU-like protein that was proposed to serve as a molecular scaffold. AtCnfU-V is constitutively expressed in several tissues of Arabidopsis, whereas the expression of AtCnfU-IVb is prominent in the aerial parts. Mutant Arabidopsis lacking AtCnfU-V exhibited a dwarf phenotype with faint pale-green leaves and had drastically impaired photosystem I accumulation. Chloroplasts in the mutants also showed a decrease in both the amount of ferredoxin, a major electron carrier of the stroma that contains a [2Fe-2S] cluster, and in the in vitro activity of iron-sulfur cluster insertion into apo-ferredoxin. When expressed in E. coli cells, AtCnfU-V formed a homodimer carrying a [2Fe-2S]-like cluster, and this cluster could be transferred to apo-ferredoxin in vitro to form holo-ferredoxin. We propose that AtCnfU has an important function and therefore is required for the biogenesis of ferredoxin and photosystem I.

KEY WORDS: iron-sulfur cluster assembly, chloroplast, photosystem I, ferredoxin