Figure 1. Structural features of RNase III and Dicer. A) Domain structure of Aquifex aeolicus RNase III (Aa-RNase III, SWISS-PROT O67082) and Homo sapiens Dicer (Hs-Dicer, SWISS-PROT Q9UPY3). Scale on top indicates the lengths of polypeptide chains; boxes in different colors represent individual domains. The red square on the endoND indicates the RNase III signature motif. B) Surface representation of the endoND dimer with the colors red and blue indicating negative and positive potential, respectively. C) Schematic view of the Aa-RNase III•dsRNA structure. The two endoNDs are shown as a molecular surface; the two dsRBDs are illustrated as ribbon diagrams (helices as spirals, β-strands as arrows, and loops as pipes) and colored in cyan and orange, respectively. The Mg2+ ions are indicated with green spheres; the two RNA strands are shown as tube-and-stick models in blue and red. endoND, endonuclease domain; dsRBD, dsRNA-binding domain; PAZ domain, an RNA-binding module found in Argonaute and some Dicer proteins (initially named for the protein families PIWI, Argonaute, and Zwilli).