CHARACTERIZATION AND TRANSCRIPTIONAL REGULATION OF THE SYNECHOCYSTIS PCC-6803 PETH GENE, ENCODING FERREDOXIN-NADP(- INVOLVEMENT OF A NOVEL TYPE OF DIVERGENT OPERATOR() OXIDOREDUCTASE )

The petH gene, encoding ferredoxin-NADP(+) oxidoreductase (FNR), has b een characterised in the unicellular cyanobacterium Synechocystis PCC 6803. Its product, FNR, was heterologously produced and functionally c haracterized. The start-site of the monocystronic petH transcript was mapped 523 bp upstream of the predicted PetH initiation codon, resulti ng in an unusually large 5'-untranslated region. The 5' end of the pet H transcript is situated within the open reading frame of phosphoribul okinase (encoded by prk), which is transcribed in opposite orientation with respect to petH. The transcription start site of the prk transcr ipt was mapped 219 bp upstream of the initiation codon, resulting in a 223 bp antisense region between both transcripts. Under many conditio ns the expression of both genes (i.e. petH and prk) is co-regulated sy mmetrically at the transcriptional level, as was concluded from both n orthern hybridization experiments and from primer extension analyses; it became uncoupled, however, when specifically petH expression was st imulated, independent of prk expression, by stressing the Synechocysti s cells with high salt concentrations. A model for a new type of bidir ectional operator, regulating the expression of petH and prk, is propo sed.