Common mechanisms for enzyme-catalyzed bisubstrate reactions

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#Unit 3. Fundamental Processes

pri-miRNA is processed in a two-step reaction

The first step is catalyzed by Drosha, an RNase III superfamily member endonuclease, in the nucleus. Drosha reduces the pri-RNA to about a 70-bp, hairpin-shaped precursor fragment, pre-miRNA

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#Unit 3. Fundamental Processes

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#Unit 3. Fundamental Processes

The two cleavages thus generate the 65-nucleotide pre-miRNA composed of the 22 bp (two helical turns) of dsRNA and the top loop.

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#Unit 3. Fundamental Processes

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#Unit 3. Fundamental Processes

First cleavage reaction in nucleus leaves a 2-nucleotide overhang on the 3’ ends of the dsRNA product. This 3’ overhang is important for recognition of that RNA molecule by the next enzyme in the cytosolic pathway, Dicer.

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#Unit 3. Fundamental Processes

The pre-miRNA liberated by Drosha is exported to the cytoplasm, where the second RNA cleavage reaction, performed by Dicer, takes place. As with Drosha, 

Dicer selects its cleavage sites using a measuring, rather than sequence-specific, mechanism.