In the effector step, the siRNA duplexes bind to a nuclease complex to form what is known as the RNA-induced silencing complex, or RISC. An ATP-depending unwinding of the siRNA duplex is required for activation of the RISC. The active RISC then targets the homologous transcript by base pairing interactions and cleaves the mRNA ~12 nucleotides from the 3' terminus of the siRNA (3, 18, 27, 29). Although the mechanism of cleavage is at this date unclear, research indicates that each RISC contains a single siRNA and an RNase that appears to be distinct from Dicer (27).
Because of the remarkable potency of RNAi in some organisms, an amplification step within the RNAi pathway has also been proposed. Amplification could occur by copying of the input dsRNAs, which would generate more siRNAs, or by replication of the siRNAs themselves (see "Possible Role for RNA-dependent RNA Polymerase" below). Alternatively or in addition, amplification could be effected by multiple turnover events of the RISC (3, 18, 27).
The Genes and Enzymes Involved in PTGS and RNAi
Possible Role for RNA-dependent RNA
Genetic screens in Neurospora, C. elegans, and Arabidopsis have identified several genes that appear to be crucial for PTGS and RNAi. Several of these, including Neurospora qde-1, Arabidopsis SDE-1/SGS-2 and C. elegans ego-1, appear to encode RNA-depen