Figure 1.

Recognition and processing of pri-miRNA by the Microprocessor complex. A representative pri-miRNA molecule is shown at the top. Pri-miRNAs are characterized by an imperfect stem of ∼3 helical turns flanked by a stretch of unstructured nucleotides at both ends, termed the basal segment and apical loop. DROSHA introduces staggered cuts ∼1 helical turn away from the basal junction and ∼2 helical turns away from the apical junction. Primary sequence motifs that are known to enhance the processing efficiency of the Microprocessor are indicated, including a basal UG motif, a flanking CNNC motif, a mismatched GHG motif, and an apical UGU/GUG motif.