The β-complex assembles on a site-specific sequence within the β-globin 3′UTR. (A) A guanine-rich region of RNA is critical for β-complex assembly. Top: EMSA analyses were conducted with K562 cytoplasmic extracts using [³²P]-labeled RNA corresponding to the full-length β-globin 3′UTR. Reactions were supplemented with competitor DNAs derived from the WT₁₅ sequence. The position of the β-complex is indicated by an arrow. Bottom: Sequences of competitor DNAs, with substituted nucleotides indicated in lower case. The corresponding region of the native β-globin 3′UTR is included for reference. (B) Site-specific mutations in the β-globin 3′UTR ablate β-complex assembly. EMSA analyses were conducted on [³²P]-labeled RNAs, corresponding to full-length wild-type and derivative β-globin 3′UTRs, using K562 cytoplasmic extract. The G-rich motif is indicated, with the specific mutations in the full-length β-globin 3′UTR depicted in lower-case type. The position of the β-complex is indicated.