FUR REGULATES THE EXPRESSION OF IRON-STRESS GENES IN THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7942

A homologue of the 'ferric uptake regulation' gene (fur) was isolated from the cyanobacterium Synechococcus sp, strain PCC 7942 by an Escher ichia coil-based 'in vivo repression assay'. The assay uses a reporter -gene construct containing the promoter region of the iron-regulated c yanobacterial gene isiA. fused to the coding region for chloramphenico l acetyltransferase. The isolated gene codes for a protein that has 41 % sequence similarity (36 % identity) to Fur from E. coil and contain s the putative iron-binding motif found in the Fur proteins of purple bacteria. No significant similarity was found to the DxtR repressor th at regulates the expression of toxin and siderophore production in Gra m-positive bacteria. Insertional mutagenesis of the cloned cyanobacter ial fur gene led to the creation of heteroallelic mutants that showed iron-deficiency symptoms in iron-replete medium, including the constit utive production of flavodoxin and of hydroxamate siderophores. Failur e to eliminate wild-type copies of the fur gene from the polyploid gen ome of Synechococcus 7942 implies that in this cyanobacterium Fur may have essential functions in addition to the regulation of genes involv ed in iron scavenging or photosynthetic electron transport.