The petH gene, encoding ferredoxin-NADP
oxidoreductase (FNR), has been characterised in the unicellularcyanobacterium Synechocystis PCC 6803. Its product, FNR, was heterologouslyproduced and functionally characterized. The start-site of the monocystronicpetH transcript was mapped 523 bp upstream of the predicted PetH initiationcodon, resulting in an unusually large 5′-untranslated region.The 5′ end of the petH transcript is situated within the open readingframe of phosphoribulokinase (encoded by prk), which is transcribed inopposite orientation with respect to petH. The transcription start site ofthe prk transcript was mapped 219 bp upstream of the initiation codon,resulting in a 223 bp antisense region between both transcripts.
Under many conditions the expression of both genes (i.e. petH and prk) isco-regulated symmetrically at the transcriptional level, as was concludedfrom both northern hybridization experiments and from primer extensionanalyses; it became uncoupled, however, when specifically petH expressionwas stimulated, independent of prk expression, by stressing theSynechocystis cells with high salt concentrations. A model for a new type ofbidirectional operator, regulating the expression of petH and prk, isproposed.