Distinct gene clusters drive formation of ferrosome organelles in bacteria.

Cellular iron homeostasis is vital and maintained through tight regulation of iron import, efflux, storage and detoxification 1-3 . The most common modes of iron storage use proteinaceous compartments, such as ferritins and related proteins 4,5 . Although lipid-bounded iron compartments have also been described, the basis for their formation and function remains unknown 6,7 . Here we focus on one such compartment, herein named the 'ferrosome', that was previously observed in the anaerobic bacterium Desulfovibrio magneticus 6 . Using a proteomic approach, we identify three ferrosome-associated (Fez) proteins that are responsible for forming ferrosomes in D. magneticus. Fez proteins are encoded in a putative operon and include FezB, a P 1B-6 -ATPase found in phylogenetically and metabolically diverse species of bacteria and archaea. We show that two other bacterial species, Rhodopseudomonas palustris and Shewanella putrefaciens, make ferrosomes through the action of their six-gene fez operon. Additionally, we find that fez operons are sufficient for ferrosome formation in foreign hosts. Using S. putrefaciens as a model, we show that ferrosomes probably have a role in the anaerobic adaptation to iron starvation. Overall, this work establishes ferrosomes as a new class of iron storage organelles and sets the stage for studying their formation and structure in diverse microorganisms.