Cellular Regulation of Iron Assimilation

Cells of plants, most microorganisms, and animals require well-defined amounts of iron for survival, replication, and differentiation. The metal is an important component of such processes as synthesis of DNA, RNA, and chlorophyll; electron transport; oxygen metabolism; and nitrogen fixation. Because of the insolubility of iron in aerobic environments at neutral and alkaline pH values, cells have had to devise specific strategies to assimilate the metal. These include (1) development of systems for reducing ferric ions to the more soluble ferrous ions at the cell surface, (2) employment of small carrier molecules (termed siderophores) that have high affinity for ferric ions and receptor proteins for the ferrated molecules, and (3) use of transferrin and other proteins that can transport ferric ions. Excessive amounts of iron are toxic, however, and intracellular storage capacity is limited and efflux mechanisms generally are lacking. Thus, cells have had to develop methods of preventing over-accumulation of the metal. These include use of (1) oxygen to convert ferrous to ferric ions, (2) small molecules that can bind ferrous ions, termed siderophraxes, and (3) proteins that, when combined with ferrous ions, repress the expression of iron transport genes. Often, one organism can prevent growth of neighbors by restricting their access to iron. In other cases, cells assist each other by sharing iron acquisition systems or by restricting influx of excess iron. Homeostatic control of other essential trace metals also is required for optimal cell function. Nevertheless, since iron thus far has received most attention, it serves as the model of mineral metabolism. Moreover, many of the observations made on control of iron metabolism suggest possible applications in prevention and management of plant and animal infections as well as of neoplastic diseases, arthropathy, and cardiomyopathy. This review will focus on (1) problems at the cellular level of iron acquisition, storage, and exclusion; and (2) the strategies devised by cells of plants, microorganisms, and animals to solve these problems.