Team:TUDelft/Modeling Cascade

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Modeling the Transcriptional Cascade

Negative cascade assembly and overview

A full description of the Transcriptional Cascade can be found here.

ODEs

The kinetic equations were written out in a Matlab script. A total of ten equations were used: one for the diffusion of IPTG into the cell, one for the binding of IPTG to LacI, four transcription equations, and four translation equations for the various levels of the cascade.

TUD eq cas.png

The notation in this system of equations can be seen in the table below:

Symbol Definition
kIPTGin, kIPTGout rate constants
k50IPTG, k50LacI, k50TetR, k50CI dissociation constants
dmRNA mRNA degradation rate
dTetR, dCI, dRFP, dGFP protein degradation rates
apLac, apTet, aλp transcription leakage (%)
cpLac, cpTet, cλp maximum transcription rates
α1, α2, α3, α4 translation rates
nIPTG, nLacI, nTetR, nCI Hill coefficients
[X]mRNA concentration of X mRNA

The solution with the default parameters of the system of ODEs can be seen below:

TUD ODE default solution.png

A function was written to determine the point at which the concentration of the final product reached a certain threshold. A threshold of 1E-8 M was used in our simulations.

Sensitivity

Parameter Normalized Sensitivity
kIPTGin, kIPTGout
k50IPTG, k50LacI, k50TetR, k50CI
dmRNA
dTetR, dCI, dRFP, dGFP
apLac, apTet, aλp
cpLac, cpTet, cλp
α1, α2, α3, α4
nIPTG, nLacI, nTetR, nCI
[X]mRNA

Parameter Sweeps

In the following plots the delay time of the cascade is shown as a function of two different parameters. A delay time off 500 is used to represent an infinite delay (maroon colour). The delay is considered to be over once the concentration of the final product (in this case GFP) goes above 10nM.

Stability

Jacobian

Design Recommendations

Based on the results of the simulations, a series of recommendations were given to the delay team to aid them in choosing parts which would maximize the delay time.

  1. Significant transcription leakages greatly shorten the delay time. Attempt to minimize leakages. Leakage of λp is a far bigger problem than pTet leakage.
  2. Use a weak promoter and a weak RBS on the last stage (λp) of the cascade.
  3. A weak pLac promoters is favorable.
  4. A strong pTet promoter is favorable.
  5. A strong RBS on CI gene is favorable.
  6. A weak RBS on TetR gene is favorable.
  7. A weak RBS on the endonuclease is favorable although a strong RBS can be used for the GFP gene.
  8. When choosing RBS and promoter strengths avoid the red areas on the stability plots.