Uncertainty Modelling and Stability Robustness Analysis of NucleicAcid-Based Feedback Control Systems

Nuno M.G. Paulino†1, Mathias Foo2, Jongmin Kim3 and Declan Bates†4

Abstract— Recent advances in nucleic acid-based chem-istry have highlighted its potential for the implementation ofbiomolecular feedback circuits. Here, we focus on a proposeddesign framework, which is able to approximate the input-output behaviour of key linear operators used in feedbackcontrol circuits by combining three elementary chemical re-actions. The implementation of such circuits using DNA stranddisplacement introduces non-linear internal dynamics due toannihilation reactions among different molecular species. Inaddition, experimental implementation of in silico designsintroduces significant levels of uncertainty and variability inreaction rate constants and equilibrium concentrations. Previ-ous work using this framework has overlooked the practicalimplications of these issues for the construction of nucleic acid-based feedback control circuits. Here, we analyse the impactof these nonlinearities and uncertainties on the stability of abiomolecular feedback loop. We show that a rigorous analysis ofits nucleic acid-based implementation requires an investigationof the associated non-linear dynamics, to decide on realisableparameters and acceptable equilibrium concentrations. We alsoshow how the level of experimental uncertainty that is toleratedby the feedback circuit can be quantified using the structuredsingular value. Our results constitute a first step towards thedevelopment of a rigorous robustness analysis framework fornucleic acid-based feedback control circuits.

I. INTRODUCTIONA primary aim of synthetic biology is the development of

systematic design and analysis frameworks for biomolecularfeedback control systems that can regulate concentrations ofmolecular species inside the cell [1]. The recently proposeduse of chemical reaction networks (CRNs), implemented vianucleic acid-based chemistry, as a programming languagefor the design of such systems [2], has made it possible toconceptualise many biological circuits in line with classicalfeedback control ideas, [3], [4].

However, a direct translation of abstract mathematicaloperators (e.g. gain, summation) into biomolecular circuitsvia chemical reactions is often not straightforward. Forexample, implementation of the subtraction operator usingCRNs generally results in a one-sided operation, (i.e. it canonly compute a positive difference between two inputs),whereas in feedback control the error signal generated by

†N. Paulino and D. Bates are with the Warwick Integrative Syn-thetic Biology Centre (WISB), School of Engineering, University of War-wick, Coventry CV4 7AL, UK 1

N.Paulino@warwick.ac.uk,4D.Bates@warwick.ac.uk

2M. Foo is with the School of Mechanical, Aerospace andAutomotive Engineering, Faculty of Engineering, Environmentand Computing, Coventry University, Coventry CV1 5FB, UKMathias.Foo@coventry.ac.uk

3J. Kim is with the Wyss Institute for Biologically Inspired En-gineering, Harvard University, Boston, Massachusetts 02115, USAJongmin.Kim@wyss.harvard.edu

the difference between the measured and the desired valuescan take both positive and negative values. This is a criticallimitation, as a one-sided error computation can lead to poorcontrol performance or even instability [5], [6].

To get around this problem, Oishi and Klavins [3] pro-posed a formalism that can perform a two-sided subtraction,by letting each signal in the circuit be the difference inthe concentration of two chemical species, x+ and x

�. Inthis way, a systematic framework for implementing simplelinear biomolecular feedback controllers can be developedusing three elementary reactions, and the same formalismhas recently been extended to include more complicatedmathematical operators [7] and non-linear controllers [8], [9].

While previous studies have indicated satisfactory stabilityand performance properties for controllers designed usingthis framework, no formal robustness analysis of thesecircuits has so far been attempted. This is an importantissue, because the implementation of CRNs using nucleicacids requires the matching of specified reaction rates forthe required biomolecular reactions to realise a particularoperator, which can be problematic in real experimentalimplementations.

The use of nucleic acids to implement CRNs is appealingbecause a simple kinetic model of toehold-mediated stranddisplacements could explain a large number of differentstrand displacement reactions from the DNA sequences [10].However, predicting reaction rate constants from sequenceinformation alone, i.e. without experimental inputs, is stillchallenging and can result in large deviations from pre-dicted design values. Improved biophysical models [11] andcomputational tools [12] can help improve the precisionof predicted kinetics of hybridization and toehold-mediatedstrand displacement. It is also reasonable to assume thatthe different reactions may be characterised and the ratesmeasured. Finally, varying the concentrations of auxiliaryspecies [2] can also be used to iteratively bring the reactionrates closer to their design values. Based on the above con-siderations, it is estimated that environment factors, togetherwith uncertainty of the characterisation, i.e., the differencebetween measured rates and the actual reaction rates in theimplementation, could result in uncertainty in the values ofrate constants of between 10% to 20%.

Thus, it is crucial to be able to formally quantify inadvance the robustness of proposed CRN-based designs tospecified levels of uncertainty. Although some formal defi-nitions of robustness in systems biology have been proposed[13], the rigorous analysis of the effects of perturbations anduncertainties in synthetic circuits calls for more structured

𝐺(𝑠)

Δ

𝑢 𝑦Fig. 1. Representation for µ-analysis, where the nominal linear systemG(s) is interconnected with the uncertainty structured in the system �.

mathematical formulations for robustness analysis, estima-tion and model-based prediction [14].

Here, we investigate the internal stability of the com-plete set of reactions resulting from an implementation ofa linear feedback control circuit using nucleic acid-basedchemistry. The assumption of high concentrations of inter-mediary species allows some simplification of the reactions,but still captures an important aspect missing from the input-output behaviour of the system: the circuit has non-linearitiesdue to the annihilation reactions, which are not observablein the input-output dynamics. We show that the presenceof these hidden internal dynamics has an impact on theequilibrium of the system, and also imposes lower boundson the concentrations of the intermediary species. Moreimportantly, the attraction of the equilibrium condition andits stabilising role also needs to be characterised. Secondly,we use the structured singular value (SSV) tool [15], [16]from robust control theory to perform a formal validationof the robustness of the proposed controller. An informalrobustness and sensitivity analysis was carried out in [3],where the system was subjected to a square wave as referenceinput over 50 simulations in the presence of random ± 10%variability in the reaction rates. This led to mismatches insome of the reactions rates, causing significant steady stateerrors in the input tracking. In contrast to simulation-basedapproaches, the SSV analysis does not rely on statistics,making it a very strong certification method as long as themodelling of the underlying uncertainty is representative.

The use of SSV, or µ-analysis, has been previously inves-tigated in systems biology [17] in an attempt to quantify andvalidate robustness in natural biochemical networks [18], andit is now widely used in other domains such as aircraft [19],[20] and space engineering [21]. Once the uncertainty in thecircuit is established, it can be aggregated in an uncertainsystem � interconnected with the nominal system (see Fig.1), representing an infinite family of systems accounting forall possible combinations of the uncertain parameters, whereany variation of the parameters will cause a deviation fromthe nominal response. The SSV analysis searches for theµ value (see e.g. [19]), which is a metric defining how farthe system is from losing stability. If the domain of � doesnot contain destabilising parameter combinations, then robuststability is guaranteed for the specified level of uncertainty.

II. UNCERTAINTY MODELLING

Fig. 2 shows the configuration of the feedback control sys-tem analysed throughout this work. It is a single-input/single-

P𝑘𝑃𝑘𝐼𝑠

+ +

𝜉1 = 𝑥1+ − 𝑥1−𝑢 𝑦-

𝜉5 = 𝑥5+ − 𝑥5−𝜉4 = 𝑥4+ − 𝑥4−

Fig. 2. The analysed feedback loop with proportional-integral controllerfrom [3].

output (SISO) system with a single state ⇠4 where the outputof a constant plant P = 1/(1+%) tracks a reference input uby the action of a proportional and integral controller. Theclosed-loop feedback trajectories evolve accordingly to:

⇠4 = k

I

⇠1 = k

I

(u� y) (1)y = P (k

P

⇠1 + ⇠4)

A. CRN Approximation of a Linear Feedback Control Circuit

Following the formalism introduced in [3], the configura-tion shown in Fig. 2 is approximated by a circuit composedof CRNs, where the negative and positive parts of a signal⇠ are represented respectively by concentrations of twodifferent species x

+ and x

�. The final value of the signal isretrieved with ⇠ = x

+ � x

�.The proposed CRNs for each of the operations in the

circuit are based on three elementary reactions, namely catal-ysis, annihilation and degradation. Here, the nomenclaturex

± represents compactly the two different species x

+ andx

�. Similarly, the notation x

± �

±

��! y

± represents compactlytwo simultaneous reactions

x

+ �

+

��! y

+, x

� �

�

��! y

� (2)

The element ; is used when the result of a reaction degrades,gets sequestered, or is removed, and no longer participatesin the reactions.

Thus, the subtraction block ⇠1 = u � ⇠5 is approximatedby the reactions:

u

± �

±1��! u

± + x

±1 , x

±5

�

±2��! x

±5 + x

⌥1 (3)

x

±1

�

±3��! ; , x

+1 + x

�1

⌘�! ;

For the weighted integrator ⇠4 = k

I

⇠1, the CRNs are:

x

±1

k

±I��! x

±1 + x

±4 , x

+4 + x

�4

⌘�! ; (4)

Finally, for the weighted summation y = ⇠5 = k

p

⇠1+⇠4, wehave:

x

±1

�

±7 k

±P����! x

±1 + x

±5 , x

±4

�

±8��! x

±4 + x

±5 (5)

x

±5

�

±9 (1+%)

�����! ; , x

+5 + x

�5

⌘�! ;

The parameters �i

, kP

, kI

, and ⌘ represent the rates of thereactions, and % is a constant (set to % = 2 as in [3]). Usingthe law of mass action, where the rate of a chemical reaction

is proportional to the product of the concentrations of thereactants (see e.g. [22]), the dynamics of the CRNs in (3)-(5) can be modelled using the ordinary differential equations(ODE):

x

+1 = ��+3 x

+1 + �

�2 x

�5 + �

+1 u

+ � ⌘x

+1 x

�1 (6)

x

�1 = ���3 x

�1 + �

+2 x

+5 + �

�1 u

� � ⌘x

+1 x

�1 (7)

x

+4 = k

+I

x

+1 � ⌘x

+4 x

�4 (8)

x

�4 = k

�I

x

�1 � ⌘x

+4 x

�4 (9)

x

+5 = ��+9 (1 + %)x+

5 + �

+7 k

+P

x

+1 + �

+8 x

+4 � ⌘x

+5 x

�5 (10)

x

�5 = ���9 (1 + %)x�

5 + �

�7 k

�P

x

�1 + �

�8 x

�4 � ⌘x

+5 x

�5 (11)

The input-output systems have states computed by the dif-ference in concentration of the species tracking the positiveand negative components of the signal with ⇠

i

= x

+i

� x

�i

.Considering the nominal case where �+

i

= �

�i

, the dynamicsof the CRNs are linear and are given by

⇠ =

2

4��3 0 ��2k

I

0 0�7kP �8 ��9(1 + %)

3

5⇠ +

2

4�1

00

3

5u (12)

With the right choice of rates �i

, and controller parametersk

I

and k

P

, the input-output behaviour of (12) mimics theinput-output behaviour of the linear system shown in Fig. 2.In this sense, the abstract CRNs of (3)-(5) can implementa similar input-output response to the feedback system (1)with biomolecular reactions, albeit with the inclusion of extradynamics.

B. Implementation with Nucleic Acids

The abstract CRNs given above can be implemented exper-imentally using DNA strand displacement (DSD) reactions[2],[3],[11]. The parameters �±

i

, k±I

, and k

±P

in the CRNsare set as functions of nominal strand displacement rates q±

i

.The individual dependencies of �±

i

on q

±i

are inferred fromthe DSD implementation in [3], and set as

�

±i

=1

2q

±i

C

max

, i 2 {1, 2, 3, 7, 8, 9} (13)

k

±I

=1

2q

±5 Cmax

, k

±P

=q

±7

q

±8

(14)

The scaling parameter C

max

results from the DSD im-plementation of the feedback system in [3], which relieson bimolecular reactions mediated by intermediary auxil-iary species with initial concentrations given by C

max

. IfC

max

� x

±i

(t), the concentration of the auxiliary speciescan be considered almost constant, and the set of reactions ofthe DSD network can be approximated by the unimolecularreactions of the CRNs in (3)-(5). Thus it is important toconsider a set of parameters where the concentrations remainsmall with respect to C

max

.The annihilation reaction ⌘ depends on a maximum

displacement rate q

max

[2], and the DSD implementationproposed in [3] results in the equivalency

⌘ =1

2q

max

(15)

If ⌘ � �

i

, the annihilation reaction keeps the concentrationsx

+i

and x

�i

minimal without changing the transmission ofthe signal ⇠

i

[3]. Often this entails that for steady state, if⇠

i

> 0 then x

+i

⇡ ⇠

i

and x

�i

⇡ 0. For ⇠i

< 0 we have theinverse with x

�i

⇡ ⇠

i

and x

+i

⇡ 0.

C. Implications of Non-Linear Dynamics

While the CRN model provides a framework to approxi-mate the dynamics in (1) using concentrations of chemicalspecies, when dealing with the DSD implementation it isimportant to note that the dynamics of ⇠

i

do not providea complete description of the system. For example, therequirement that x

±i

(t) ⌧ C

max

cannot be verified fromthe input-output response alone, since the concentrations ofx

i

are not observable in the states ⇠i

, or y.In addition, the stability and steady states of the sys-

tem cannot be inferred from (12) alone, due to the ab-sence of the non-linear annihilation reactions. Consider thenon-negative state vector of species concentrations x =⇥x

+1 , x

�1 , x

+4 , x

�4 , x

+5 , x

�5

⇤T 2 R+

0 . The system of ODE’sin (6)-(11) can be represented compactly as

x = Ax+Bu� ⌘g(x) (16)

The input vector u = [u+, u

�]T contains both positive and

negative components of the reference input, and the linearpart of the model is given by

A =

2

6666664

��

+3 0 0 0 0 �

�2

0 ��

�3 0 0 �

+2 0

k

+I 0 0 0 0 00 k

�I 0 0 0 0

�

+7 k

+P 0 �

+8 0 ��

+9 (1 + %) 0

0 �

�7 k

�P 0 �

�8 0 ��

�9 (1 + %)

3

7777775

B =

2

666664

�

+1 00 �

�1

0 00 00 00 0

3

777775, C = [ 0 0 0 0 1 �1 ]

A non-linear term g(x) is now present containing themultiplications due to bimolecular annihilation reactions:

g(x) =

2

6666664

x

+1 x

�1

x

+1 x

�1

x

+4 x

�4

x

+4 x

�4

x

+5 x

�5

x

+5 x

�5

3

7777775

As mentioned above, the states ⇠i

2 R are obtained fromthe six non-negative components x

±i

2 R+0 . This subtraction

results in the projection of the trajectories x

±i

in a R3

manifold, defined by the projection matrix : R6 ! R3

=

2

41 �1 0 0 0 00 0 1 �1 0 00 0 0 0 1 �1

3

5 (17)

Applying this projection to the non-linear system in (16) wehave that · g(x) = 0, and the trajectories of ⇠

i

result fromthe linear dynamics

⇠ = (Ax+Bu� ⌘g(x)) =1

2 A T

⇠ + Bu (18)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.95 1 1.05 1.10

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Fig. 3. Probability density function fitted to the distribution of theequilibrium estimation (1 + e6) = x

0+5 /r5 (solid black), and the limits

(dotted red) containing 99.9% of the respective cumulative distributionfunction (dashed red).

An immediate consequence is that the steady state of (18)tells us nothing about the steady state of the DNA strandconcentrations, which are the solution to (16).

The other significant consequence is that, in our example,while 1

2 A T is Hurwitz, the matrix A is not. The stability

of the experimental implementation using DSD depends alsoon ⌘g(x) and the equilibrium solution. For example, withinitial conditions x(0) = 0, at t = 0 the system in (16)is unstable (g(x) = 0 and A is not Hurwitz). The non-linear term ⌘g(x) is an adaptive stabilising term, wherethe state x needs to converge to an equilibrium wherethe system becomes stable. By collapsing the trajectorieswith the projection (17), this information about the internalstability is hidden.

D. Equilibria Change with Uncertainty on the Parameters

Our objective is to measure the robustness of the system touncertainty. The strand displacement rates q

±i

depend on thedesigned DNA toehold sequences [10], which can be hard topredict. Even after experimental characterisation, there willalways be an error between the measured and real value ofthe reaction rates. Here, we consider that the reaction ratesare never perfectly known and are therefore uncertainties inthe system.

Considering the start of the time response to be whenu

+ = u

� = 0, the equilibrium condition x

0 is the non-trivialsolution to the non-linear equation

⌘ · g(x0) = Ax

0 (19)

Thus the solution depends on the parametrisation of A

and the strand displacement rates q

±i

. Since it dependsdirectly on the uncertain parameters, the solution is a movingequilibrium, [23], that changes with the values of the rates.

However, through numerical simulation of the dynamics in(16) it is possible to produce an arbitrary number of pairs as-sociating an equilibrium condition x

0 with the correspondingvalues of q±

i

, opening the way for estimation techniques. TheSSV analysis framework requires rational representations of

uncertainty, and therefore each of the equilibrium compo-nents must be approximated by a polynomial function of therates. To do this, the 14 uncertain rates q

±i

are organised ina column vector q, and an estimator is obtained through thelinear function

x

0 = r(q) + " = V q + " (20)

The samples of x

0 and q are arranged respectively ascolumns of R and Q, and V = RQ

T

�QQ

T

��1 is theoutcome of a Least Squares (LS) fitting. The fit of eachof the components is further improved by expressing the LSfitting error "

i

, i 2 {1, . . . , 6} as a quadratic function ofr

i

(q):

r

i

(q) = r

i

(q) +⇣↵

i

+ �

i

r

i

(q) + ⇢

i

(ri

(q))2⌘+ #

i

(21)

This new function reduces quadratic distortions of the errorsand "

i

< #

i

. The fittings of the matrix V and the threecolumn vectors ↵, �, and � in (21) were done using 10000random samples of q

±i

in the intervals in Table I, to arriveat the estimation function and error

x

0(q) = r(q)(1 + e) (22)

Despite the presence of a small estimation error, we havenow computed a rational function that relates the uncertainparameters and the equilibrium state.

To account for the estimation error we add correspond-ing uncertainties, as suggested in [23], obtained from thedistributions of the fitting errors for each component e

i

.These were fitted with a probability density function (p.d.f.).Limits of variability for e

i

were defined containing 99.9%of the cumulative density function to define six additionaluncertainties, which cover the error in tracking the movingequilibria x

0i

= r

i

(q) · �ei

. The resulting limits of alluncertainties �

ei

in the presence of 13% of uncertainty onq

±i

are listed in Table I. See the case of error (1 + e5) inFig. 3 when estimating x

0+5 .

Linearising (16) around the equilibrium point (x, u) =(x0

, 0) then allows for a frequency representation of thesystem’s response around its equilibrium, which is suitablefor analysis within the SSV framework. The error dynamics

TABLE IUNCERTAIN SET � COMPRISED BY THE REACTION RATES qi AND THE

FITTING ERROR OF THE EQUILIBRIUM �ei .

Name Nominal Variabilityq

±i ,i 2 {1, 2, 3, 5, 7, 8, 9} 800 [13, 13]%

�e1 1 [0.967081 1.09241]�e2 1 [0.967098 1.09217]�e3 1 [0.962463 1.06614]�e4 1 [0.962197 1.06822]�e5 1 [0.961924 1.09484]�e6 1 [0.96421 1.10709]

0 0.5 1 1.5 2 2.5 3 3.5 4105

-5

0

5 10-9

0 0.5 1 1.5 2 2.5 3 3.5 4105

-5

0

5 10-9

Fig. 4. Top: reference input u = u

+ � u

� and its two components.Bottom: output of the projected CRN in (18) (dashed black) approximatesthe response of the ideal linear model in (1) (blue).

around equilibrium x

0 are

x

e

=@f

@x

����x

0

x

e

+@f

@u

����x

0

u

e

=�A� ⌘J(x0)

�x

e

+Bu

e

(23)y

e

= x

+5e � x

�5e (24)

where x

e

= x � x

0, ue

= u � u

0, and y = y

e

� y

0. TheJacobian matrix J(x0) is given by

J(x0) =@g(x)

@x

����x

0,0

=

2

6666664

x

0�1 x

0+1 0 0 0 0

x

0�1 x

0+1 0 0 0 0

0 0 x

0�4 x

0+4 0 0

0 0 x

0�4 x

0+4 0 0

0 0 0 0 x

0�5 x

0+5

0 0 0 0 x

0�5 x

0+5

3

7777775(25)

The function in (22) is used to include � in the Jacobianthrough J(r

i

(q) · �ei

) to account for the moving equilibriumconditions. In this way, the equilibrium becomes correlatedwith the uncertainty, as in the original non-linear system.This step is important in order to remove the conservatismthat would be introduced by assuming an independent vari-ability of the equilibrium conditions.

III. ANALYSIS RESULTSThe model in (16) for the non-linear system was imple-

mented and simulated in Matlab/Simulinkr, and analysedwith the Robust Control Toolbox from Mathworks [24].

In our example, Cmax

= 1µM and the displacement ratesq

i

= 800M�1s

�1 (as in [3]). The maximum rate was setto q

max

= 106M�1s

�1. The reference input u(t) is givenby a square wave oscillating between ±4 ⇥ 10�9

M . Thedesired alternating step input is composed of the differencebetween two concentrations in the presence of the annihila-tion reaction u

+ +u

� ⌘�! ; (see Fig. 4). The trajectories x

±i

for the complete nominal system in (16) are shown in Fig. 5

0 0.5 1 1.5 2 2.5 3 3.5 4105

0

510-9

0 0.5 1 1.5 2 2.5 3 3.5 4105

0

0.5

1

10-8

0 0.5 1 1.5 2 2.5 3 3.5 4105

0

2

410-9

Fig. 5. Trajectories of the states for the non-linear system in (16), withinitial conditions around the equilibrium solution of (19).

when the initial conditions are set to x

0. The tracking error⇠1 = x

+1 �x

�1 can assume positive and negative values and is

null at steady state. The trajectories of x±1 are not inferable

from ⇠1, where x

+1 and x

�1 are always positive, and alternate

their role with each new step on the reference.For the robustness analysis we consider that variability

is present for all 14 strand displacement reaction ratesindependently, which can allow for q

+i

6= q

�i

. The nominaland uncertain values for q

±i

and the fitting errors �

ei

aresummarised in Table I. The final system has a high repetitionof uncertainties, and for strictly real uncertainties it canbe computationally hard to obtain good lower bounds onthe SSV (= 1/µ). However, it is sufficient to demonstratethat the upper-bound of the SSV is less than 1 for allfrequencies to guarantee that the system is stable for allpossible parameterisations (see e.g. [19]).

As shown in Fig. 6, for uncertainty in the reaction ratesof up to 13%, the SSV analysis guarantees that the closed-loop system is stable for all uncertain parameters shown inTable I, since the upper bound on 1/µ remains below 1 forall frequencies. For levels of 14% or more the analysis isnot conclusive. Since the fitting errors �

ei

introduce conser-vatism, which does not exist in the non-linear system wherethe equilibrium is always the true value, a better descriptionof r(q) would improve the accuracy of the bound. Evenwith this extra artificial conservatism, however, our analysisshows that the system is guaranteed to be robustly stablefor a 13% variation in the reaction rates, which falls withinthe expected 10 � 20% level of experimental uncertainty.This represents crucial information to help experimentalistsestimate the accuracy with which reaction rates need to beimplemented for successful construction of these circuits.

IV. CONCLUSIONS

In this paper we investigated the problem of designinglinear feedback control circuits that can be practically im-

10-7 10-6 10-5 10-4 10-3 10-2

0.5

1

1.5

13%

14%15%

16%

17%

Fig. 6. Upper bounds on destabilising 1/µ, for different levels ofuncertainty. The upper bounds are lower than 1 across all frequencies foruncertainty levels of 13% or lower.

plemented using nucleic acid-based chemistry. We showedhow the implementation of even simple linear circuits withdistinct chemical species depends on non-linear terms thatdrive important dynamics which are not observable in thesystem’s input-output response. Rigorous analysis of therobustness of such circuits requires that we focus on thedynamics of the complete state vector and analyse theinternal stability of the system. This provides informationabout closed loop stability and the system’s steady state thatis highly relevant when choosing experimental parameters.

The use of the SSV framework provides a method for therigorous quantification of how much the circuit implemen-tation deviates from the ideal response in the presence ofexperimental uncertainty, and allows a certification of thestability of the system around its equilibrium conditions. Toachieve a linear representation, it was necessary to expressthe equilibrium conditions as a function of the uncertainty,to account for shifts due to parameter variability. We foundthe system to be robustly stable around its equilibrium forexperimentally feasible uncertainty levels in the reactionrates. Further work is ongoing to investigate the significanceof the non-linear terms in the dynamics of more complexfeedback circuits, to improve the estimation of the movingequilibrium, to include additional sources of uncertainty,to quantify the effects of uncertainties on performance,and to obtain improved estimates of likely ranges of theuncertainties based on experimental data.

ACKNOWLEDGEMENTFunding from the EPSRC & BBSRC (Centre for Doctoral

Training in Synthetic Biology EP/L016494/1, and WarwickIntegrative Synthetic Biology Centre BB/M017982/1).

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