A molecular characterisation of PsaN: a nuclear-encoded subunit of the photosystem I complex of Chlamydomonas reinhardtii.

Photosystem I (PSI) is a membrane protein complex composed of a large number of polypeptide subunits, designated PsaA to PsaN. In eukaryotes, the genes for these subunits are distributed between the nuclear and chloroplast genomes with the core subunits encoded by chloroplast genes. The function of many of the small nuclear-encoded subunits is unknown. Primarily, we are investigating the role of a nuclear-encoded subunit PsaN, which is absent in prokaryotic PSI complexes. PsaN is an extrinsic subunit located entirely on the lumenal side of the thylakoid membrane. We have isolated and sequenced the psaN gene in Chlamydomonas and shown that it is a single copy gene with an unusual exon structure. We are currently generating nuclear PSI mutants by random mutagenesis, in order to isolate tagged Chlamydomonas mutants disrupted in psaN or the other nuclear genes. The isolation of such mutants will provide an insight into their function and a valuable null background for structure-function studies, in which site-directed variants of the gene are re-introduced into the nuclear genome. In addition, we are analysing an existing psaF-deficient mutant of Chlamydomonas reinhardtii [1] in order to determine the function of the C-terminal region of the PsaF subunit. [1] Farah, J. et al. (1995). EMBO J. 14: 4976-4984.