We have constructed and analysed a cyanobacterial mutant that lacks the putative homologue of ycf37, the chloroplast open reading frame 37, which is conserved in different algae, but missing in the plastome of higher plants. In this report we show that Ycf37 of Synechocystis sp. PCC 6803 contains three tetratrico-peptide repeat (TPR) units resembling the structural organization of Ycf3, a protein that has been suggested to function as a chaperone during photosystem (PS) I complex formation. We demonstrate a light-activated transcript accumulation of this gene. Inactivation of ycf37 leads to a lower PSI/PSII ratio and a higher phycocyanin/chlorophyll ratio in Synechocystis cells. The observed alterations in the ycf37 mutants and the structural organization of the gene product suggest a functional role in PSI stability or assembly.

Abbreviations used: Chl, chlorophyll; PC, phycocyanin; PS, photosystem; TPR, tetratrico-peptide repeat; ORF, open reading frame.

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