Inactivation of the conserved open reading frame ycf34 of Synechocystis sp. PCC 6803 interferes with the photosynthetic electron transport chain

Ycf34 is a hypothetical chloroplast open reading frame that is present in the chloroplast genomes of several non-green algae. Ycf34 homologues are also encoded in all sequenced genomes of cyanobacteria. To evaluate the role of Ycf34 we have constructed and analysed a cyanobacterial mutant strain. Inactivation of ycf34 in Synechocystis sp. PCC 6803 showed no obvious phenotype under normal light intensity growth conditions. However, when the cells were grown under low light intensity they contained less and smaller phycobilisome antennae and showed a strongly retarded growth, suggesting an essential role of the Ycf34 polypeptide under light limiting conditions. Northern blot analysis revealed a very weak expression of the phycocyanin operon in the ycf34 mutant under light limiting growth in contrast to the wild type and to normal light conditions. Oxygen evolution and P700 measurements showed impaired electron flow between photosystem II and photosystem I under these conditions which suggest that the impaired antenna size is most likely due to a highly reduced plastoquinone pool which triggers regulation on a transcriptional level. Using a FLAG-tagged Ycf34 we found that this protein is tightly bound to the thylakoid membranes. UV–vis and Mössbauer spectroscopy of the recombinant Ycf34 protein demonstrate the presence of an iron–sulphur cluster. Since Ycf34 lacks homology to known iron–sulphur cluster containing proteins, it might constitute a new type of iron–sulphur protein implicated in redox signalling or in optimising the photosynthetic electron transport chain.

► Cyanobacterial homologue of the chloroplast ORF ycf34 is a novel iron–sulphur protein.
► Ycf34 is required for growth under light limitation.
► Synechocystis mutants lacking Ycf34 are impaired in phycocyanin accumulation.
► Inactivation of ycf34 alters the photosynthetic electron transport chain.