Fig. 5.

Comparison between the Fano factors of nascent and mature RNA. Contour plot showing the variation of $\mathrm{\Xi }$—a measure of the difference between the two Fano factors which is defined in Eq. (18)—with the non-dimensional parameters $T_g$ and $T_m$ which denote the ratio of the mean elongation time to the timescales of promoter switching and decay of mature RNA, respectively. As can be appreciated from Eq. (17), $\mathrm{\Xi }$ is positive if the Fano factor of nascent RNA is larger than that of mature RNA and negative if the reverse is true. The line $T_m\approx 1-\frac{5}{8}{T}_g$, where $\mathrm{\Xi }=0$, shows where the two Fano factors are identical