Figure 4. The effects of the critical determinants on the three systems at steady state.

We examined the effects of the critical determinants (i.e., β_TE3 and K_TE3) on the steady-state outputs of the three systems under the activating signal (S = 1.0). E3 was perturbed by continuously varying (a) β_TE3 and (b) K_TE3 such that E3 was initially upregulated and subsequently downregulated. The decrease of β_TE3 leads to the downregulation of E3, and thereby the horizontal axis of (a) is set in a descending order (from 2.0 to 0). (c) The regulation of E3 by T was removed to observe the system response with respect to the steady-state level of E3 (E3_ss). The K_{m_E3T} and K_E3P represent the binding affinities (or the reaction intensities) of E3 for proteolysis and transcription, respectively. (d) The curves of the steady-state level of P (P_ss) generated by varying K_{m_E3T} under the activating signal (S = 1.0). (e) Identical curves generated by varying K_XP. K_TP2 and K_IP are the kinetic parameters that correspond to K_E3P of ITUD in SNFL and DTUD, respectively.