Regulation of zinc-responsive Slc39a5 (Zip5) translation is mediated by conserved elements in the 3'-untranslated region

Translation of the basolateral zinc transporter ZIP5 is repressed during zinc deficiency but Zip5 mRNA remains associated with polysomes and can be rapidly translated when zinc is repleted. Herein, we examined the mechanisms regulating translation of Zip5. The 3'-untranslated region (UTR) of Zip5 mRNA is well conserved among mammals and is predicted by mFOLD to form a very stable stem-loop structure. Three algorithms predict this structure to be flanked by repeated seed sites for miR-328 and miR-193a. RNAse footprinting supports the notion that a stable stem-loop structure exists in this 3'-UTR and electrophoretic mobility shift assays detect polysomal protein(s) binding specifically to the stem-loop structure in the Zip5 3'-UTR. miR-328 and miR-193a are expressed in tissues known to regulate Zip5 mRNA translation in response to zinc availability and both are polysome-associated consistent with Zip5 mRNA localization. Transient transfection assays using native and mutant Zip5 3'-UTRs cloned 3' to luciferase cDNA revealed that the miRNA seed sites and the stem-loop function together to augment translation of Zip5 mRNA when zinc is replete.