Relative to
ferredoxin:NADP(+) reductase (FNR) from chloroplasts, the comparable
enzyme in cyanobacteria contains an additional 9 kDa domain at its amino-terminus. The domain is homologous to the
phycocyanin associated linker
polypeptide CpcD of the light harvesting
phycobilisome antennae. The phenotypic consequences of the genetic removal of this domain from the petH gene, which encodes FNR, have been studied in Synechocystis PCC 6803. The in frame deletion of 75 residues at the amino-terminus, rendered chloroplast length FNR
enzyme with normal functionality in linear photosynthetic electron transfer.
Salt shock correlated with increased abundance of petH
mRNA in the wild-type and mutant alike. The truncation stopped salt stress-inducible increase of
Photosystem I-dependent cyclic electron flow. Both photoacoustic determination of the storage of energy from
Photosystem I specific far-red light, and the re-reduction kinetics of P700(+), suggest lack of function of the truncated FNR in the
plastoquinone-
cytochrome b(6)f complex
reductase step of the PS I-dependent cyclic electron transfer chain. Independent
gold-immunodecoration studies and analysis of FNR distribution through activity staining after native
polyacrylamide gelelectrophoresis showed that association of FNR with the thylakoid membranes of Synechocystis PCC 6803 requires the presence of the extended amino-terminal domain of the
enzyme. The truncated DeltapetH gene was also transformed into a
NAD(P)H
dehydrogenase (NDH1) deficient mutant of Synechocystis PCC 6803 (strain M55) (T. Ogawa, Proc. Natl. Acad. Sci. USA 88 (1991) 4275-4279). Phenotypic characterisation of the double mutant supported our conclusion that both the
NAD(P)H
dehydrogenase complex and FNR contribute independently to the
quinone cytochrome b(6)f
reductase step in PS I-dependent cyclic electron transfer. The distribution, binding properties and function of FNR in the model cyanobacterium Synechocystis PCC 6803 will be discussed.