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How Microorganisms Transport Iron: In the midst of plenty, microorganisms are often in danger of iron-starvation; the mechanism by which they transport iron has now been elucidated.

Iron is a component of many important enzyme systems in virtually every type of living cell. Given the immense abundance of this element in the earth's surface-it is exceeded only by aluminum, silicon, and oxygen-one might expect that cells would face few problems in fulfilling their requirements. Wrong. Ever since photosynthetic organisms started pumping oxygen into the atmosphere 2 billion years ago, available iron has been converted to its ferric form, which is virtually insoluble. Living cells have therefore had to go to extraordinary lengths to obtain this vital metal. The means by which some of them do it has finally been worked out in considerable detail. By exploiting the process that the gut bacterium Escherichia coli sometimes uses in becoming a virulent pathogen in other parts of the body, Albrecht Bindereif and Joe Neilands of the University of California, Berkeley, have been able to dissect the genetic system that underlies the elaboration of this iron-transport machinery. They are also able to speculate on how the system is regulated at the molecular level. Very simply, microorganisms use the same general approach for getting iron into the cell at anything more than a residual rate. This high-affinity system has two parts: the production of an ironbinding molecule, which is excreted from the cell; and the elaboration of a membrane receptor molecule, which binds the iron complex and transports the metal to the cell's interior. What the Berkeley biochemists have shown is that the membrane receptor and the enzymes that synthesize the iron-binding molecule are coded for by five genes in one operon, which is turned off when iron levels in the cell have been satisfied. The off switch appears to be a complex formed between iron and a polypeptide (a repressor protein), which is coded for at a separate location from the operon. The fact that no other metal requires a transport mechanism as elaborate as this one emphasizes the very great problems to be overcome in getting iron into the cell in the face of very great need. Iron can get into cells without the aid of this system, but the rate of this low-affinity system is minuscule and the mechanism obscure.