Characterization of a cyanobacterial iron stress-induced gene similar to psbC

Recently we have reported that the flavodoxin gene from the cyanobacterium Anacystis nidulans R2 is transcribed as part of an iron stress-induced operon containing multiple mRNA species (D. E. Laudenbach, M. E. Reith, and N. A. Straus, J. Bacteriol. 170: 258-265, 1988). Here we report that nucleotide sequence analyses of DNA located immediately upstream of the flavodoxin gene revealed an open reading frame of 1,026 bases (designated isiA; iron stress inducible) with a deduced amino acid sequence showing similarity to that of the psbC polypeptide of higher plants and cyanobacteria. Assuming proteolytic cleavage of the initial methionine residue, the open reading frame encodes a 341-amino-acid polypeptide with a molecular mass of 36,824 daltons. Amino acid sequence comparisons with known psbC polypeptides from spinach and A. nidulans R2 showed extensive similarity, especially in the proposed membrane-spanning regions. Mung bean nuclease mapping and primer extension experiments have localized a transcriptional start site to a position 19 bases upstream from the first methionine codon of the isiA gene product. The upstream region contains an Escherichia coli-like -10 sequence but lacks the typical -35 consensus sequence. Approximately 15, 25, and 150 bases upstream from the isiA transcription start site are 17 base sequences which resemble the operator sequences of iron-regulated genes of E. coli.