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Summary Ferredoxin:NADP oxidoreductases (FNRs) constitute a family of flavoenzymes that catalyse the exchange of electrons between ferredoxin and NADP(H). In cyanobacteria FNR provides NADPH for photoautotrophic metabolism, but the enzyme is also capable of oxidizing NADPH providing reduced ferredoxin. In the cyanobacterium Synechocystis sp. strain PCC6803, the unique petH gene has two translation products depending on growth conditions. As a consequence two isoforms of the FNR accumulate – FNRL and FNRS. In the present work, analysis of petH expression reveals that different transcriptional start points (tsp) are responsible for this differential translation initiation. Under standard conditions (where FNRL accumulates), two tsps were found at −52 and −34 relative to the first translation start site. Under nitrogen-starvation conditions (where FNRS accumulates) a tsp was mapped at −126 relative to the first translation start site. Therefore, the transcript responsible for FNRS translation is longer than that producing FNRL. In addition, expression of the short or long transcript in E. coli resulted in the accumulation of FNRL or FNRS respectively. This result demonstrates that translation can initiate at two different sites, 336-bases apart (ATG-1 to ATG-113), depending only on the 5′UTR structure. |
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