Core components of the RNAi
Dicer family proteins contain an N-terminal helicase
domain, a C-terminal segment containing dual RNase III
domains, and one or more dsRNA-binding motifs. Family
members also contain a PAZ domain.
•Member of RNAseIII family of enzymes.
• Recognize and process dsRNA into siRNA.
• Dicer family proteins are ATP-dependent nucleases.
• Dicer homologs exist in many organisms
includingC.elegans, Drosophila, yeast and humans.
• miRNA Biogenesis
– Transcribed from endogenous gene as pri-miRNA
• Primary miRNA: long with multiple hairpins
• Imperfect internal sequence complementarity
– It is processed into 70-nt hairpins by the RNase III family
member Drosha to become the pre-miRNA.
• Note: How does it identify pri-miRNA?
– Hairpin terminal loop size
– Stem structure
– Hairpin flanking sequences
– The pre-miRNA is exported to the cytoplasm by Exportin 5.
– It is cleaved by the R2D2/Dicer heterodimer into the mature
• Symmetric 2nt 3’ overhangs, 5’ phosphate groups
Comparison of Mechanisms of MiRNA Biogenesis and Action
Better complementarity of MiRNAs and targets in plants.
Summary of differences between plant and animal MiRNA
miRNA genes: 100-200 100-500
Location in genome: intergenic regions Intergenic regions, introns
Clusters of miRNAs: Uncommon Common
MiRNA biosynthesis: Dicer-like Drosha, Dicer
Mechanism of repression mRNA cleavage Translational repression
Location of miRNA
target in a gene: Predominantly Predominantly the 3′-UTR
the open-reading frame
sites in a target gene: Generally one Generally multiple
Functions of known
target genes: Regulatory genes Regulatory genes—crucial
crucial for development, for development, structural
enzymes proteins, enzymes