Regulation of Gene Expression by Iron

Abstract

Iron serves as a central regulatory substance which controls the expression of almost 40 genes in Escherichia coli. Why is this so? Iron, with the exception of some lactobacilli, is required by all organisms in rather high concentrations (between 10⁵ and 10⁶ ions per microbial cell) [1] since it is contained in the reaction centers of many redox enzymes in the cytoplasm and the cytoplasmic membrane. Depending on the protein environment, Fe²⁺ /Fe³⁺ spans the unusually large standard redox potential range from +300 to -500 mV, which makes it an ideal redox cofactor. Despite the great abundance of iron in nature, iron supply poses great problems for organisms growing under aerobic conditions. Fe³⁺ occurs at pH 7 as a hydroxyaquo polymer with a free Fe³⁺ concentration in the order of 10^-18 M (10³ ions per ml). This is certainly not enough to supply 10⁹ bacteria per ml with 10¹⁴ Fe³⁺ ions per generation. To fulfill their iron demand, bacteria developed elaborate iron supply systems which include iron complexing substances, called siderophores, and Fe³⁺-siderophore transport systems. Synthesis of the siderophores and the transport systems are subject to iron control. The enzymes and transport proteins involved are synthesized under conditions of iron starvation. In addition, a number of bacterial toxins are formed at low iron supply. As far as these toxins damage eukaryotic cells, bacteria may gain access to intracellular iron stores, so that the toxins also contribute to the bacterial iron supply.