Both subunits of rat liver ferritin are regulated at a translational level by iron induction.

A few hours after administering iron to rats, liver ferritin synthesis increases several fold. However, Northern blot analysis with cDNA probes for ferritin light (L) and heavy (H) subunit mRNAs failed to show an increase in total population of either messenger. Cytoplasmic distribution of ferritin messages was therefore investigated in control and iron administered rats killed at 3.5 hours. The liver post-mitochondrial supernatant was fractionated on a sucrose gradient to separate polyribosomes, monosomes, ribosomal subunits and cell sap. RNA extracted from each fraction and analyzed using Northern blotting showed that 65% of the total mRNA population for each subunit was present in the cell sap of control rats, presumably as mRNP particles since ribosomal RNA was absent from this fraction. After iron administration, these reserves of free mRNA were recruited onto the polysomes, reducing the free mRNA pool to 15% of the total. We interpret this to be due to activation of blocked ferritin messages on entry of iron into the cell.