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

Functional investigations on gene products of conserved chloroplast ORFs in the cyanobacterium Synechocystis sp. PCC 6803. Ulf Duehring^*,1, Klaus-Dieter Irrgang², Ali Alawady¹, Sandra Mikolajczyk¹, Annegret Wilde¹, ¹ Institute of Biology, Humboldt University Berlin, Chausseestr. 117, Berlin, Germany² Max-Volmer Laboratories, Technical University Berlin, Strasse des 17. Juni 135, Berlin, Germany

ABSTRACT- Chloroplast genomes mainly contain genes involved in photosynthesis and housekeeping. The remaining genes include open reading frames of unknown function and have been designated ycf for hypothetical chloroplast open reading frame. No final conclusion can be drawn about these ycfs without functional analysis. Some of these ycfs are highly conserved in all plastid genomes of higher plants and algae. Ycf37, Ycf45 and Ycf53 are hypothetical chloroplast open reading frames that occur only in a limited number of algal plastid genomes. However, similar sequences have been identified in the nuclear genomes of plants, e.g. Arabidopsis and rice. Nearly all ycfs are also present in cyanobacteria supporting the idea that the gene products have highly conserved functions. We report here on functional analysis of the cyanobacterial ycf37, ycf45 and ycf53 gene products using mutants generated in Synechocystis sp. PCC 6803. We show that Ycf37 is a new thylakoid membrane protein possibly involved in PSI biogenesis. Ycf45 mutants express IsiA and assemble an IsiA-PSI trimer complex under long term high-light conditions. Ycf53 shows sequence similarity to the Arabidopsis nuclear gene GUN4. Arabidopsis GUN4 encodes a putative regulatory protein that interacts with Mg chelatase and stimulates its activity. In Synechocystis 6803 this mutation seems to be lethal. The possible role of Gun4 from in the Mg as well as the Fe porphyrin branch in tetrapyrrole biosynthesis will be discussed.

KEY WORDS: photosystem I, IsiA, Synechocystis 6803, gun4