Localization and function of ferredoxin:NADP + reductase bound to the phycobilisomes of Synechocystis

Each phycobilisome complex of the cyanobacterium Synechocystis PCC 6803 binds ∼2.4 copies of ferredoxin:NADP + reductase (FNR). A mutant of this strain that carries an N‐terminally truncated version of the petH gene, lacking the 9 kDa domain of FNR that is homologous to the phycocyanin‐associated linker polypeptide CpcD, assembles phycobilisome complexes that do not contain FNR. Phycobilisome complexes, consisting of the allophycocyanin core and only the core‐proximal phycocyanin hexamers from mutant R20, do contain a full complement of FNR. Therefore, the binding site of FNR in the phycobilisomes is not the core‐distal binding site that is occupied by CpcD, but in the core‐proximal phycocyanin hexamer. Phycobilisome complexes of a mutant expressing a fusion protein of the N‐terminal domain of FNR and green fluorescent protein (GFP) contain this fusion protein in tightly bound form. Calculations of the fluorescence resonance energy transfer (FRET) characteristics between GFP and acceptors in the phycobilisome complex indicate that their donor–acceptor distance is between 3 and 7 nm. Fluorescence spectroscopy at 77K and measurements in intact cells of accumulated levels of P700 + indicate that the presence of FNR in the phycobilisome complexes does not influence the distribution of excitation energy of phycobilisome‐absorbed light between photosystem II and photosystem I, and also does not affect the occurrence of ‘light‐state transitions’.