Figure 1.

PTR under high efficiency of RISC-formation. Fano factors (main graphs) and average molecular numbers per cell (insets) of the free small RNAs m (yellow), loaded RISCs A_m (blue), and free target transcript R (red) are plotted as functions of the target gene transcription rate k_R under low competition PTR conditions (M₀ = 1/600). (Colored y-axis lines and labels refer to corresponding quantities plotted.). Small RNAs have a fixed rate of production k_m = 1/min, which determines the derepression threshold (8). (A) Stoichiometric PTR. RISC and substrate degradation occurs in a short range around k_R = k_m, and the transition from repressed to derepressed targets is sharp. (B) Catalytic PTR at sixfold increased k_R range. Argonautes are titrated by the highly efficient substrate throughout the k_R range because RISCs are not degraded with the targets, leading to strong target repression.