Two essential FtsH proteases control the level of the Fur repressor during iron deficiency in the cyanobacterium S ynechocystis sp. PCC 6803

Summary The cyanobacterium S ynechocystis sp. PCC 6803 expresses four different FtsH protease subunits ( FtsH 1‐4) that assemble into specific homo‐ and heterocomplexes. The FtsH 2/ FtsH 3 complex is involved in photoprotection but the physiological roles of the other complexes, notably the essential FtsH 1/ FtsH 3 complex, remain unclear. Here we show that the FtsH 1 and FtsH 3 proteases are involved in the acclimation of cells to iron deficiency. A mutant conditionally depleted in FtsH 3 was unable to induce normal expression of the IsiA chlorophyll‐protein and FutA 1 iron transporter upon iron deficiency due to a block in transcription, which is regulated by the Fur transcriptional repressor. Levels of Fur declined in the WT and the FtsH 2 null mutant upon iron depletion but not in the FtsH 3 downregulated strain. A similar stabilizing effect on Fur was also observed in a mutant conditionally depleted in the FtsH 1 subunit. Moreover, a mutant overexpressing FtsH 1 showed reduced levels of Fur and enhanced accumulation of both IsiA and FutA 1 even under iron sufficiency. Analysis of GFP ‐tagged derivatives and biochemical fractionation supported a common location for FtsH 1 and FtsH 3 in the cytoplasmic membrane. Overall we propose that degradation of the Fur repressor mediated by the FtsH 1/ FtsH 3 heterocomplex is critical for acclimation to iron depletion.