MicroRNAs (miRNAs) are widespread posttranscriptional regulators of gene expression. They are processed from longer primary transcripts that contain foldback structures (reviewed in). In animals, a complex formed by Drosha and DGCR8/Pasha recognizes the transition between the single-stranded RNA sequences and the stem loop to produce the first cleavage step in miRNA biogenesis. Whereas animal precursors are of uniform size and shape, their plant counterparts comprise a collection of variable stem loops, and little is known about the structural clues recognized during their processing. Here, we designed an unbiased approach based on the random mutagenesis of the MIR172a precursor to study miRNA processing in plants. Randomly mutated precursors were overexpressed in Arabidopsis, and their activity was determined in vivo. We gathered sequence data from these transgenes and used it to build a MIR172a precursor map highlighting relevant and neutral positions for its processing. A 15 nucleotide stem segment below the miRNA/miRNA(*) duplex was essential for MIR172a processing. In contrast, mutations in the terminal-loop region were mostly neutral, yet a loop was required for miR172 biogenesis. The results could be extended to other precursors, suggesting the existence of common features in at least part of the plant precursors.
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