Genetic screen yields two unique drosha alleles that are defective in miRNA biogenesis. (A) Schematic of Drosha protein indicates two RNase III domains (RIII) and double-stranded RNA binding domain (gray square), as well as locations of mutations. The drosha∆,E859K allele deletes 147 amino acids (∆) and contains a missense mutation at residue 859. (B–E) Eye discs from third-instar larvae containing WT (B), droshaR662X (C), droshaR1113X (D), and drosha∆,E859K (E) clones stained with anti-Drosha antibodies (magenta). Clones are indicated by the absence of GFP. (Scale bar, 10 μm.) (F) Northern blots of total RNA from third-instar larvae of indicated genotypes probed for miR-2b, let-7, miR-125, and U6 snRNA (loading control). (G) In vitro processing of pri-let-7 by WT, E859K, ∆, E859K+∆, or dominant negative (DN) Drosha proteins. Length of incubation is indicated in minutes.