mRNA destabilization is the dominant effect of mammalian microRNAs by the time substantial repression ensues (gene expression)

MicroRNAs (miRNAs) regulate target mRNAs through a combination of translational repression and mRNA destabilization, with mRNA destabilization dominating at steady state in the few contexts examined globally. Here, we extend the global steady-state measurements to many additional mammalian contexts and find that regardless of the miRNA, cell type, growth condition or translational state, mRNA destabilization explains most (70% to >90%) miRNA-mediated repression. We also determine the relative dynamics of translational repression and mRNA destabilization for endogenous mRNAs as a miRNA is induced. Although translational repression occurs rapidly, its effect on gene expression is relatively weak, such that by the time consequential repression ensues, the effect of mRNA destabilization dominates. These results add to the fundamental understanding of miRNAs, imply that consequential miRNA-mediated repression is largely irreversible and simplify future studies, dramatically extending the known contexts and time points for which monitoring mRNA changes captures most of the direct miRNA effects.