Repression by Fur is not the main mechanism controlling the iron-inducible isiAB operon in the cyanobacterium Synechocystis sp. PCC 6803

The iron deficiency-dependent regulation of isiAB transcription in Synechocystis sp. PCC 6803 was analyzed by fusion of modified isiAB promoter fragments to gfp and in vivo quantification of Gfp fluorescence. For the putative Fur box only a slight repressing impact on promoter activity could be shown. In a heteroallelic fur mutant a corresponding incomplete repression of isiAB transcription under iron-replete conditions confirmed the role of Fur in isiAB regulation. However, a 90 bp region upstream of the putative −35 box of the isiAB promoter was essential for full promoter activity under iron-deplete conditions. This pattern indicates a dual promoter regulation by both a repressing mechanism exhibited via the Fur system and an unknown activating mechanism.