Expression of a gene encoding a novel ferredoxin in the cyanobacterium Synechococcus 6301

A gene was discovered in the cyanobacterium Synechococcus 6301 that encodes a protein highly related to members of the [2Fe-2S] ferredoxin family found in chloroplasts and cyanobacteria. It follows a cluster of seven genes encoding subunits of the cyanobacterial ATP synthase complex. It is transcribed as a monocistronic mRNA of 408 nucleotide residues. Transcription starts at a site 55 bp upstream of the initiator methionine codon. Transcriptional initiation and termination signals with sequences similar to those found in Escherichia coli are not present. Comparison of the predicted sequence of the ferredoxin protein with those of other cyanobacterial and plant ferredoxins shows an average sequences identity of about 40%. Twelve amino acid residues are invariant, including the four cysteine residues that provide ligands for the [2Fe-2S] cluster. The deduced Synechococcus ferredoxin protein sequence has a C-terminal extension of eight amino acid residues relative to most other 2Fe-2S ferredoxins except for those from halobacteria, which also have a C-terminal extension. The sequence of the Synechococcus protein is most closely related to ferredoxins from the two complex cyanobacteria Chlorogloeopsis fritschii and Mastigocladus laminosus. The deduced protein sequence is not that of the major soluble ferredoxin that has been isolated from Synechococcus 6301 and is reported in the accompanying paper [Wada, Masui, Matsubara & Rogers (1988) Biochem. J. 252, 571-575]. So it appears to be a novel [2Fe-2S] ferredoxin and Synechococcus 6301 contains at least two [2Fe-2S] ferredoxins, which may have different roles in vivo.