LOOPS AND BULGE LOOPS IN IRON-RESPONSIVE ELEMENT ISOFORMS INFLUENCE IRON REGULATORY PROTEIN-BINDING - FINE-TUNING OF MESSENGER-RNA REGULATION/

A family of noncoding mRNA sequences, iron-responsive elements (IREs), coordinately regulate several mRNAs through binding a family of mRNA- specific proteins, iron regulatory proteins (IRPs). IREs are hairpins with a constant terminal loop and base-paired stems interrupted by an internal loop/bulge tin ferritin mRNA) or a C-bulge tin m-aconitase, e rythroid aminolevulinate synthase, and transferrin receptor mRNAs). IR P2 binding requires the conserved C-G base pair in the terminal loop, whereas IRP1 binding occurs with the C-G or engineered U-A. Here we sh ow the contribution of the IRE internal loop/bulge to IRP2 binding by comparing natural and engineered IRE variants. Conversion of the inter nal loop/bulge in the ferritin-IRE to a C-bulge, by deletion of U, dec reased IRP2 binding by >95%, whereas IRP1 binding changed only 13%. Mo reover, IRP2 binding to natural IREs with the C-bulge was similar to t he Delta U-6 ferritin-IRE: >90% lower than the ferritin-IRE. The resul ts predict mRNA-specific variation in IRE-dependent regulation in vivo and may relate to previously observed differences in iron-induced fer ritin and m-aconitase synthesis in liver and cultured cells. Variation s in IRE structure and cellular IRP1/IRP2 ratios can provide a range o f finely tuned, mRNA-specific responses to the same (iron) signal.