Team:USTC/Project

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Revision as of 20:04, 21 October 2009

USTC
Home Team Project Modeling Parts Standard & Protocol Software Tool Human Practice Notebook

Team:USTC/Project

Contents


The Background

Evolution vs. Design

Evolution and design are two sides of the coin.

Until 150 years ago, people believed that we are designed and created by the God, a god, gods or other intelligent designers. After Charles Darwin's On the Origin of Species in 1859, more and more people began to realize that a god is not necessary. During the process of evolution, we can be created automatically, without any intelligence.

Design and engineering are processes with the need of knowledge. Before we can design a machine, we have to know everything about the possible parts. If any information is unknown, we have to learn it. If nobody knows it, we have to measure it by ourselves. If all the knowledge needed are too much, we have to collaborate with others. Computer-aided design have to be used to accomplish more complex tasks. Space shuttle and microprocessor are seen as the most complex systems engineered, but they are far too simple when compared with the complexity of biological systems created by evolution.

On the other side, evolution is a completely different process. Variation is random, selection is directional based on the fitness to the environment, that is all. However, all the amazing things on our planet is emerged from this simple process, no more thing is needed.

Why? All the complexity is solved by this simple process without any input of knowledge?

Yes. The answer is on the scale. The evolution process is so simple that it can be scaled up infinitely. The complex problem can be solved in the distributed evolution system. Any success in the distributed system can be amplified by the selection process. Therefore, although the variation is random, it will be powerful enough to search for the solutions, when the scale of the system is big enough.

In the design process, the variation is directional based on the knowledge, but the process is not scalable, because of its requirement of intelligence. As a result, it is much more difficult to solve complex problems by design.


Directed Evolution

Although Darwin's theory was published 150 years ago, people have used the power of evolution more than ten thousand years. Domestication of plants and animals is done by artificial selection, which is the process of intentional breeding for certain traits, therefore changing the direction of evolution.

Directed evolution is a method using the similar principle to create new biomolecules with desired properties. The targets of directed evolution include enzymes, antibodies, aptamers, ribozymes, biosynthetic pathways, and synthetic genetic circuits. More information can be found on the Frances H. Arnold research group, the Ellington lab, and the Szostak lab.

The directed evolution experiment contains several rounds of 3 steps: variation, selection, and amplification. Variation is the mutation or recombination of the information encoded in the DNA, usually by error-prone PCR and DNA shuffling respectively. Selection is the process of separating the variants with desired phenotypes from others, it can either refer to screening (isolate good variants) or selection (eliminate bad variants), either in vivo or in vitro. Amplification is the replication of the variants after selection, which recovers the population size for the new round of directed evolution.

New function of biomolecules or biological systems is difficult to be rationally designed, but directed evolution is successfully used in solving these problems.


Design & Evolutionary Approaches in iGEM Projects

Flow Chart of Engineering a Genetically Engineered Machine

In the iGEM competition, teams specify, design, build, and test simple biological systems made from standard, interchangeable biological parts. Most projects engineer the systems by modeling and measurement, including this project itself. This engineering approach is proved to be feasible but difficult in the design of biological systems during this years.

Most BioBrick parts have never been characterized, because characterization and documentation of the parts usually takes a lot of time. Further more, the properties of BioBrick parts and devices are very sensitive to the conditions they work in, such as strain, plasmid, other parts used together, culture medium, growth rate, temperature, pH, shaking rate, light, and so on. It is very difficult to characterize all the parameters, so the parts often have to be remeasured in different projects before the behavior of the system can be predicted.

There are also many projects using evolutionary approaches. These approaches use selection or screening to find the needed part, eliminate the steps of measurement or constructing new parts with desired properties.

Some of the teams using evolutionary approaches in their project:
Team Name / People Competition / Lab Project Approach Wiki Link
DavidsoniGEM 2006Solving the Pancake Problem with an E. coli Computerin vivo Recombination; SelectionDavidson
Missouri_WesterniGEM 2006Solving the Pancake Problem with an E. coli Computerin vivo Recombination; SelectionMissouri_Western
AlbertaiGEM 2007Plan B: Building Butanol BioBricksMutagenic Compound; SelectionAlberta
Boston_UniversityiGEM 2007Increasing the Current Output of Microbial Fuel Cells Through the Directed Evolution of Shewanella oneidensisError-Prone PCR; ScreeningBoston_University
Davidson_Missouri_WiGEM 2007Living Hardware to Solve the Hamiltonian Path Problemin vivo Recombination; ScreeningDavidson_Missouri_W
HarvardiGEM 2007Cling-E. coli: Bacteria on targetScreeningHarvard
USTCiGEM 2007Extensible Logic Circuit in BacteriaError-Prone PCR; ScreeningUSTC
Calgary_SoftwareiGEM 2008EvoGEM: An Evolutionary Approach to Designing LifeEvolutionary AlgorithmCalgary_Software
ETH_ZurichiGEM 2008Make yourself simpler, stupid! Or how engineering a self-minimizing cell leads to the Minimal GenomeControlled expression of restriction enzymes and ligases in vivo; SelectionETH_Zurich
IIT_MadrasiGEM 2008StressKit: A BioBrick library of Lac-repressed σ24, σ28, σ32 and σ38 promoters for Escherichia coliScreeningIIT_Madras
IllinoisiGEM 2008Cell-based and in vitro antigenic sensors for medical diagnosticsError-Prone PCR; ScreeningIllinois
Newcastle_UniversityiGEM 2008A Computational Intelligence Approach to Developing a Diagnostic Biosensor: The Newcastle BugBusters ProjectEvolutionary AlgorithmNewcastle_University
Peking_UniversityiGEM 2008A genetic circuit to direct evolution of proteins in vivoin vivo Mutagenesis; ScreeningPeking_University
Tokyo_TechiGEM 2008Coli.Touch – implementation of a pressure-responsive genetic circuit in E. ColiError-Prone PCR; ScreeningTokyo_Tech
WarsawiGEM 2008Bacterial device for creating and production of interactors for any given bait proteinin vivo Mutagenesis; SelectionWarsaw
Jason R. KellyEndy LabScreening plasmidScreeningEndy:Screening_plasmid

Evolutionary Biology, Population Genetics & Evolutionary Algorithm


Problems to Be Solved


The Blueprint

The Goal


Modules & Flow Chart of the System

Modules & Flow Chart of E.ADEM

Scoring Function


Self-Adaptive Controller


Variation Function


Selection Function


Repoter


The Prototype

What to Do First?


Constitutive Promoter Family as Stimulus Signals

We choose to use constitutive promoters instead of the conditional operon impressions (represented by IPTG-induced expression) as the stimulus signals to test the system. The stimulus signals in a testing system are supposed to be definite and stable. However, the IPTG-induced signals are susceptible to many environmental factors. The process of inducible expression involves a series of dynamic actions in physical chemistry: the diffusion process of IPTG molecules, and the equilibruim between the attachment and disattachment of IPTG to the promoter. That way, the expression signals would fluctuate in a large scope in experiments and the mathematic analyses would be very complicate.

Comparatively, the stimulus signals based on a series of constitutive promoters of different levels are far stabler since the process are relatively direct.

  • The signals produced by the constitutive promoters can maintain at the steady state during the measurement.
  • Constitutive-promoter expressions can give out several different stimulus signals in one system without any disturbation among them. That is, several constitutive promoters can work independently in one system to produce double or triple stimulus signals. Instead, the IPTG-induced testing system can imput only one signal at a time corresponding to the concentration of the IPTG.
  • Two or more systems with different stimulus signals can grow in the same nurture if the signals are produced by the constitutive promoters. For example, in our project, several kinds of E.coli with different imputs signals can grow in the same culture medium preparing for the screen.
We have characterized the constitutive promoter family in detail and the measurements and the results are described in the registered parts[1].


Design of the Self-Adaptive Controller

Design of the self-Adaptive Controller

The self-adaptive controller consists of two quorom sensing parts one tetR parts and a hybrid promoter.

Two quorom sensing systems

There are two quorom sensing parts in the self adaptive controller. LuxR is expressed by a constitutive promoter and interacts with AHL produced by both LuxI in two parts. The complex then activates hybrid promoter. LuxI in different parts perform different functions. LuxI's expression in Quorom sensing 1 depends on the population density and take effect in evolution in the aspect of cell density. LuxI in part 2, on the other hand, is expressed correspondent to the evolution score directly. Thus it takes part in the evolution process directly.

TetR parts

TetR is expressed correspondent to the evolution score, just like the LuxI from quorom sensing part 2. However tetR represses the hybrid promoter. High score means high tetR level. High tetR level means low death rate.


Principle of Operation

  • Properties of selection system

The selection system consists of hybrid promoter and ccdB. AHL-LuxR complex and tetR input perform contrast effect. We perform several experiments to identify how the hybrid promoter work. The property is as shown below:

Example.jpg

  • How different "score" results in evolution

When high score and low score E.coli are grown seperately:

High.jpg

Low.jpg

What if we mix them together:

Mix.jpg

Vector & Chassis

PSB1A3: high copy number

Top10

MD


Assembly Road Map

In order to keep the whole process in perspective, we designed maps to direct our work in wet lab.

Assembly Road Map_1: Milestone1.
Assembly Road Map_2: Milestone2.

The Progress