Iron availability dramatically alters the distribution of ferritin subunit messages in Drosophila melanogaster

Insect ferritins have subunits homologous to the heavy and light chains of vertebrate ferritins. Cloning and sequence of the heavy chain homologue (HCH) of Drosophila melanogaster ferritin subunit have been reported earlier. When Northern blots of D. melanogaster RNA were probed with a cDNA for this HCH, three bands were observed. It was shown that these represented at least four classes of mRNA of various lengths. The polymorphism results from alternative splicing of an intron in the 5' untranslated region (UTR) that contains the iron-responsive element (IRE) and from two alternative polyadenylation sites in the 3' UTR. This has also been reported by others [Lind, M. I., Ekengren, S., Melefors, O. & Söderhäll, K. (1998) FEBS Lett. 436, 476-482]. By hybridizing Northern blots with specific probes, it has been shown that the relative proportions of the messages vary with the life stage and especially with iron supplementation of the diet. Iron significantly increases the amount of ferritin HCH messages and dramatically shifts the balance toward those messages that lack an IRE and/or have a short 3' UTR. In the larvae this change takes place in the gut, but not in the fat body. We speculate that this dramatic shift in message distribution may result from an effect of iron on the rate of transcription or message degradation, or from an effect on the splicing process itself. Synthesis of ferritin HCH subunit mRNAs that lack an IRE may be important under conditions of iron overload.