Small RNAs generated in vitro by budding-yeast Dicer

The Dicer ribonuclease III (RNase III) enzymes process long double-stranded RNA (dsRNA) into the small interfering RNAs (siRNAs) that direct RNA interference. Here, we describe the structure and activity of a catalytically active fragment of Kluyveromyces polysporus Dcr1, which represents the noncanonical Dicers found in budding yeast. The crystal structure reveals a homodimer that resembles bacterial RNase III but includes a novel N-terminal domain and newly identified catalytic residues conserved throughout eukaryotic RNase III enzymes. Biochemical analyses show that Dcr1 dimers bind cooperatively along the dsRNA substrate and cleave at precise intervals based on the distance between consecutive active sites. Thus, unlike canonical Dicers, which successively remove siRNA duplexes from the dsRNA termini, Dcr1 initiates processing in the interior and works outward. The distinct mechanism of budding-yeast Dicers establishes a novel paradigm for natural protein-based molecular rulers and imparts substrate preferences with ramifications for biological function. High-throughput sequencing of small RNAs generated by in vitro processing of long dsRNA using Kluyveromyces polysporus Dicer