TRANSCRIPTIONAL ANALYSIS AND PROMOTER MAPPING OF THE FDXA GENE WHICH ENCODES THE 7FE FERREDOXIN (FDII) OF RHODOBACTER-CAPSULATUS

The structural gene (fdxA) coding for ferredoxin II (FdII) of the photosynthetic bacterium Rhodobacter capsulatus has been previously cloned and sequenced. Transcription of the fdxA gene was studied by mRNA analyses and by use of plasmid-borne fdxA::lacZ translational fusions. The transcription start site was mapped 23 bp upstream of the initiation codon, as deduced from analysis of mRNA by mung bean nuclease protection and primer extension experiments. A motif ressembling the canonical sequence observed in sigma 70-dependent Escherichia coli promoters is present at the expected distance (-10/-35) from the proposed transcription start site. mRNA analysis by Northern hybridization revealed a fdxA-specific transcript of approximately 0.4 kb, indicating that fdxA is transcribed as a single gene. fdxA expression, as measured by the activity of the fdxA::lacZ fusion, was found to be constant during growth and reached a similar level under all growth conditions tested. These results suggest that FdII is constitutively synthesized in R. capsulatus.