THE FERRITIN GENES - THEIR RESPONSE TO IRON STATUS

Iron is a required nutrient which, at high concentrations, can peroxid ize cell lipids and other cellular components. To prevent excess iron from damaging cells, it is stored in ferritin, which consists of a she ll of protein subunits of two related types, H (heavy) and L (light), surrounding a cavity in which the iron can be deposited. In order to p repare for a rapid increase in ferritin in response to a rise in cellu lar iron, a large number of dormant ferritin mRNAs are accumulated in the cytoplasm. These can be rapidly activated to yield a large populat ion of ferritin subunits. Regulation is achieved through a 28-nucleoti de ''stem-and-loop'' structure near the beginning of the H- and L-ferr itin mRNAs. When this structure is associated with a binding protein ( iron regulatory element binding protein, IRE-BP), translation of the f erritin mRNA cannot proceed. However, when intracellular iron accumula tes, IRE-BP releases its hold and translation of the mRNA then takes p lace. IRE-BP has been identified as a cytosolic form of aconitase, con taining several fourfold iron-sulfur clusters. Within each cluster one iron atom is labile; this may be the mechanism by which IRE-BP respon ds to intracellular iron levels. Finally, transcription of the L- and H-genes shows that L is preferentially transcribed in response to incr eased iron intake, whereas H responds to cell differentiation and othe r factors. More work is needed to define independent transcription of the individual genes, including regulation of components other than th e 28-nucleotide segment.