A unifying model for microRNA-guided silencing of messenger RNAs.

Silencing by the miRNA-guided RNA induced silencing complex (miRISC) is dependent on Ago2-chaperoned base pairing between the miRNA 5' seed (5'S) and a complementary sequence in the 3' untranslated region of an mRNA. Prevailing mechanistic understanding posits that initial 5'S pairing can further allow functional base pair expansion into the 3' non-seed (3'NS), while functionally distinct non-canonical pairing was reported between only the 3'NS and the mRNA coding sequence. We developed single-molecule kinetics through equilibrium Poisson sampling (SiMKEPS) to measure highly precise binding and dissociation rate constants of varying-length target sequences to 5'S and 3'NS in a paradigmatic miRISC isolated from human cells, revealing distinct stable states of miRISC with mutually exclusive 5'S and 3'NS pairing. Our data suggest conformational rearrangements of the Ago2-bound miRNA that regulate alternative 5'S- and 3'NS-driven target recognition. The resulting model reconciles previously disparate observations and deepens our acumen for successfully marshaling RNA silencing therapies.