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Timer Plus Sumo
From 2013.igem.org
Timer Plus Sumo
In this section the system of Figure 1 is modeled. The structure of the timer has two repressing promoters (PcI and Ptet) and the input is the T7 promoter and the output is the protease Ulp-1. This Ulp-1 cleaves off the SUMO from the produced SUMO-peptide.
Figure 1: Circuit of the timer including sumo cleaving
Differential Equations
The above circuit can be represented by the following differential equations. We assume a binary behavior of the T7 promoter. In the presence of IPTG, the T7 promoter will be active. So, we make the assumption that the T7 is binary variable with two possible states: either active 1 or inactive 0.
Parameters
| Parameter | Value | Description | Units | Reference |
| ca | 1020 | Translation rate per amino acid | min-1#a-1 | [7] |
| cT7 | 4.16 | Maximum transcription rate of T7 | #m/min | [2] |
| cptet | 2.79 | Maximum transcription rate of Ptet | #m/min | [4] |
| cci | 1.79 | Maximum transcription rate of Pci | #m/min | [3] |
| dmRNA | 0.231 | Degradation rate of mRNA | min-1 | [8] |
| dTET | 0.1386 | Degradation rate of TET | min-1 | [9] |
| dCI | 0.042 | Degradation rate of CI | min-1 | [9] |
| dPEP | 6.3*10-3 | Degradation rate of the peptide | min-1 | Assumed the same as GFP |
| dPSU | 6.3*10-3 | Degradation rate of the peptide plus SUMO | min-1 | Assumed the same as GFP |
| dUlp | 1.263*10-2 | Degradation rate of Ulp | min-1 | Assumed twice the rate of GFP |
| lt7 | 0.002 | Leakage factor of T7 | - | Assumption |
| lptet | 0.002 | Leakage factor of Ptet | - | Assumption |
| lci | 0.002 | Leakage factor of Pci | - | Assumption |
| ktet | 6 | Dissociation constant of Ptet | #m | [10] |
| kci | 20 | Dissociation constant of Pci | #m | [10] |
| kcUlp | 3 | Turnover rate of Ulp | min-1 | [6] |
| nci | 3 | Hills coefficient | - | [11] |
| ntet | 3 | Hills coefficient | - | [11] |
| s | 0 or 1 | Activation/Inactivation of T7 promoter | Binary | Assumption |
| sci | 228 | Length of CI | amino acids | [12] |
| sPSU | 18 + 110 | Length of peptide plus SUMO | amino acids | [12] |
| sTET | 206 | Length of TET | amino acids | [13] |
| sUlp | 233 | Length of Ulp1 | amino acids | [13] |
Results
Here the results are given of the simulation upon activating the T7 promoter. For starting conditions the steady state values of the concentrations are used when T7 is switched off.
Figure 2: Simulation Results
Discussion
In the graph the effect of the timer is clearly observed, at first the peptide+SUMO is produced, and after approximatly 80 minutes more Ulp is produced leading to an increased cleaving of the SUMO. Also note that there is cleaving of the SUMO already from the start, as the promoters are somewhat leaky.
