Fig. 2. C. elegans lin-4 and let-7 miRNA family members and 3'UTR interactions. (A) Examples of heteroduplexes between lin-4 and two out of seven binding sites in the lin-14 3'UTR and the single binding site in the lin-28 3'UTR (Moss et al., 1997; Wightman et al., 1993). The duplexes are imperfect and overall duplex structure varies because of binding site sequence variations; however, the 5' seed region is usually paired. (B) Representative heteroduplexes demonstrating conservation of let-7-binding sites in the 3'UTRs of lin-41 homologs in worm (Ce), flies (Dm) and zebrafish (Zf) (Pasquinelli et al., 2000; Slack et al., 2000). (C,D) Alignments of let-7 and lin-4 family members. Residues identical to C. elegans let-7 or lin-4 are shown in red. The 5' seed region, which is important for binding-site selection, is underlined. (C) Alignment of the four worm (Ce) let-7 miRNA family members with let-7 genes from Drosophila (Dm) and human (Hs). Only a subset of known human let-7 miRNAs is shown (Lim et al., 2003). (D) Alignment of lin-4 miRNA family members (Ambros et al., 2003; Lim et al., 2003).