Proposed mechanisms of transcriptional activation or repression of the hepcidin gene by USF1/USF2 and their dominant-negative mutants. (A) A simplified depiction (not to scale) of the exon-intron arrangement of Usf2 and its deletion mutants. E1 to E10 are the 10 exons of the gene. BR indicates basic DNA-binding domain; HLH, helix-loop-helix domain; and LL, leucine zipper domain. The DNA-binding domain of Usf2 encoded by E7 has been interrupted (▾) in the Paris Usf2 knockout mouse and in the dominant-negative mutant USF2ΔB. (B) Model of transactivation or repression. In transactivation, USF2 (expressed upstream of hepcidin) forms heterodimers with USF1 through their HLH-ZIP domains. Following recruitment of coactivator complexes (eg, the adenovirus E1A-binding protein p300/CREB-binding protein [CBP]), the heterodimer binds to the E-boxes (designated E-1 to E-4) to drive hepcidin transcription (top panel). However, in the presence of a dominant-negative mutant (A-USF or USF2ΔB), USF2 (or USF1) may form stable but transcriptionally crippled heterodimers (shown with a clip) that are unable to recruit coactivator complexes; hepcidin transcription is therefore stalled (bottom panel). *USF2 stop codon.