Figure 3.

(A) pre-rRNA precursor and cleavage sites for 18S rRNA production. (B) Structural model of early pre-40S ribosomes, viewed from the subunit interface. Only Rps required for early steps are depicted [*27]. H44 is highlighted in blue. Nucleotides that form base pairs with U3 snoRNA and ITS1 are shown in red or yellow spacefill, respectively. In early pre-ribosomes the top of H44, which otherwise covers the U3-interacting regions, is disrupted and might be dislocated to allow access to U3 snoRNA [*45]. The nucleotides that form base pairs with the yellow region in H44 are directly downstream of cleavage site A₂ [*45], which allows the rough location of that site. The 5′- and 3′-ends of 18S rRNA (sites A₁ and D) point toward the back, as indicated with arrows. Bms1 and Rcl1 are positioned where Tsr1 binds, and near the A₂ site, respectively. U3 snoRNA binds the red regions in 18S rRNA, as well as regions in the 5′-ETS, schematically depicted as a red box, and for clarity is not explicitly shown. Mrd1 is positioned to bind near the U3 binding site, as well as near UtpB, in keeping with interactions with both ([58], see Figure 2). The UtpB complex, shown in lavender, forms interactions with the U3/Mpp10 complex, and Enp1 and Ltv1 [36,37,40,41,62], located at the back of the beak [**28,52,*53].