Ferredoxin:NADP oxidoreductase (FNR), encoded by the petH gene, provides NADPH for CO2 fixation in photoautotrophic cells and oxidizes NADPH in heterotrophic cells. Whereas there is only one petH gene copy in the cyanobacterium Synechocystis sp. PCC6803, two FNR isoforms accumulate (FNRL and FNRS). It was proposed that FNRL fulfills functions in linear electron transport while FNRS is involved in cyclic electron transport and respiration. In addition, FNRS was shown to be the product of an internal translation initiation within the FNRL open-reading frame. During my PhD, I revealed the mechanism by which petH translation leads to the accumulation of either one of the FNR isoforms. I showed that each isoform is produced from a specific mRNA; under standard conditions -when FNRL accumulates- two mRNAs carrying similar leaders (32 and 53 bases) are transcribed; while under nitrogen starvation -when FNRS accumulates- an mRNA, carrying a longer leader (126 bases), is transcribed. Transcriptional fusions of the Escherichia coli lac promoter to petH, with different leader sequences, suggest that the translation regulation does not require a specific factor; but rather to a spontaneously occurring secondary structure, adopted by the longer leader. Such a structure could activate FNRS translation initiation and prevent that of FNRL. Toeprinting assays confirmed this hypothesis in vitro; translation initiation complexes locations were mapped to the FNRL initiator codon in the short mRNA and to the FNRS initiator codon in the longer mRNA. Thus we have uncovered a novel gene-regulation mechanism by which two isoforms are produced from a single gene in Synechocystis sp. PCC 6803.
