Theory

In this section we will review the theory behind our approach to establishing a flow through a microtube.

Phototaxis

Background

We want to be able to control the amount of flow in the tube through a remote signal. The signal we have chosen is light, since light does not have any effect on the composition of the fluid. This means that the probability of unwanted chemical interactions is reduced. Having looked at previous iGEM work on light sensitive systems, which have all been focused on transcriptional regulation, we realised that we would need a different approach for the fast response times our system requires. We have therefore focused our work on photorhodopsins that integrate into the chemotaxis pathway, giving us very fast response to light stimulation.

The type of light that we will use is blue light, which functions as a repellent in our case. This will make the bacteria want to get away from the light source which in turn results in an increased tumbling frequency, why will be explained a little further down this text. Since we chose E. coli as our model organism and wanted to use a light signal, we would have to increase it's sensitivity to bluelight, which naturally is very, very small. Through research we found out that the Halobacterium salinarum has a very well researched phototaxis mechanism, where the individual membrane domains role in the process had been solved and transferred to E. coli. Which means that we would have to pick up on that research and create this mechanism as biobricks.

The following model shows the way we want to couple the phototaxis pathway to E. coli's natural chemotaxis pathway. This is almost identical to the phototaxis pathway in H. salinarum except that the HtrII is directly coupled to CheA, so that there is no Tsr involved.



The way the halobacterial pathway works is that the photonreceptor is a protein called sensory rhodopsin II, which absorbs the blue light and in response changes it's conformation. HtrII is just a transducer and signals this to CheA, which in turn gets phosphorylated and afterwards passes the phosphate group on to CheB. Phosphorylated CheB binds to the flagellar motor switch, so that the flagella starts rotating clockwise, which induces the tumbling motility pattern. The more CheY gets phosphorylated the higher the tumbling frequency will be. Our focus is to get this working in E. coli, which craves a few extra steps. First we will have to link the SopII and HtrII domains together with a 9 amino acid residue linker, so that the signal transducing happens succesfully in E. coli. We also have to fuse HtrII and Tsr in their cytoplasmic domains, which is the HAMP domain, that both proteins contain. Fusion in this HAMP domain effectively couples the phototaxic receptors to the chemotaxis pathway, so that a phototactic effect is possible in E. coli. These construction informations were obtained from the article: An Archaeal Photosignal-Transducing Module Mediates Phototaxis in Escherichia coli by Spudich et Al. [http://jb.asm.org/cgi/content/full/183/21/6365]. That this system is functional in vitro in E. coli has also been shown by Spudich et Al in the article Photostimulation of a Sensory Rhodopsin II/HtrII/Tsr Fusion Chimera Activates CheA-Autophosphorylation and CheY-Phosphotransfer in Vitro† [http://www.ncbi.nlm.nih.gov/pubmed/14636056].
We will only have to add retinal to the system, which is needed for proper function of the fusion,chimera-protein. Therefore we want E. coli to produce retinal on its own, by transferring the gene for the enzyme that cleaves beta-carotene to retinal from flies (drosophila melanogaster). For the accumulation of beta-carotene we will use the biobrick BBa_K274210, which was constructed by the Cambridge team in 2009 [1]. We will expand this brick's functionality by coupling it with the enzyme that cleaves beta-carotene to retinal. In that way we will be able to construct a retinal generator with the help of Cambridge's and our part. Here is a model of the retinal generator:



In the end we want to split the whole fusion, chimer into two biobricks that can be fused as a composite part. By doing this we hopefully introduce biobricks that give E. coli phototaxic abilities and also introduce modularity into the complex, so that its signalling function can be coupled to other pathways than chemotaxis.

BioBrick design

SopII-HtrII-Tsr fusion,chimer coding sequence: BBa_K343003 (Sandboxed)

Retinal production

Background

Retinal biosynthesis

Retinal is produced naturally in many species. Our construct uses genes from the plant pathogen Pantoea ananatis and from the fruit fly, Drosophila melanogaster.

In our construct, part of the synthesis is carried out by the BioBrick BBs_K274210, created by the Cambridge team in 2009. It consists of four genes crtE, crtB, crtI and crtY from Pantoea ananatis that together make up the pathway that converts farnesyl pyrophosphate to beta-carotene, which is a precursor for retinal.

• crtE encodes the protein geranyl-geranyl pyrophosphate synthase that converts farnesyl pyrophosphate to geranyl-geranyl pyrophosphate by elongating it by one unit of isopentenyl.
• crtB encodes the protein phytoene synthase and synthesizes phytoene by putting together two molecules of geranyl-geranyl pyrophosphate whilst cutting off 2 molecules of pyrophosphate.
• crtI encodes the protein phytoene dehydrogenase and converts phytoene to lycopene by converting the trans bond to a cis bond and adding more cis double bonds.
• crtY encodes the protein lycopene B-cyclase and converts lycopene to beta-carotene.

The pathway (including the step that generates retinal) is summed up below:

To introduce the final step from beta-carotene to retinal, we use the gene ninaB from Drosophila melanogaster. This gene encodes the protein beta-carotene 15,15’-monooxygenase, which cleaves beta-carotene to produce two molecules of retinal under the consumption of oxygene.
We have inserted the part BBa_343006 into a different plasmid from the BBa_K274210 part because both parts are very long and thus wouldn’t have fitted in one plasmid.

The effect of retinal on the system

Characterizing the retinal BioBrick

We characterized our BioBrick using two methods: Photospectrometry and high performance liquid chromatography (HPLC). While characterizing our own construct, we simultaneously characterized the Cambridge part, BBa_K274210 seeing as we wish to find out if the two parts work in concert.
Seeing as both beta-carotene and retinal have unique and characteristic spectres when studied by UV-vis spectrometry, this is a good place to start. The spectra can be obtained by harvesting beta-carotene- and retinal-producing cells, resuspending them in acetone and lysing them, which we chose to do by sonication. Afterwards, the cell debris can be pelleted and the supernatant can be examinated.
The acetone suspension of beta-carotene and retinal can then be subjected to photospectrometry, and the obtained values and spectra can be compared to those of pure beta-carotene or retinal in known concentrations. This method provides both a qualitative answer to whether or not the desired compound is present and a qualitative indication of the concentration in the cells.
This is the method by which the Cambridge team characterized their beta-carotene-generating BioBrick in 2009.
Apart from photospectrometry, the resuspension of beta-carotene or retinal in acetone can be subjected to analysis by HPLC. HPLC can be used to separate retinal from beta-carotene, to get a better indication of whether or not our retinal-generating part actually produces retinal from beta-carotene. For this particular purpose, we use a C-18 column, and the eluents used are as follows:
A-buffer: 100% methanol with 0,1% trifluoroacetic acid.
B-buffer: A mixture consisting of 60% methanol and 40% acetone with 0,1% trifluoroacetic acid.
Due to the chemical properties of beta-carotene and retinal, respectively, retinal will come through the column before beta-carotene when a gradient is run from 100% A-buffer to 100% B-buffer.
Afterwards, the solutions of purified retinal or beta-carotene are studied using UV-vis photospectrometry and the values and spectra are compared to those of the same compounds of known concentrations. Again, this gives both qualitative and quantitative indications of whether the compund in question is present and, if it is, in what cncentration.
See our results here.

BioBrick design

Retinal generator biobrick: BBa_K343002 (Sandboxed)

Further use of the retinal BioBrick

Hyperflagellation

Background

The flagella regulon in E. coli is composed of at least 50 genes organized in no less than 14 operons that all contribute to the synthesis and operation of flagella. The operons are synthesized in a three-level transcriptional cascade where the FlhDC operon is the master regulator at the top of the cascade. The flagella regulon is tightly controlled by nutritional and environmental conditions, E. coli starved of amino acids showed temporarily decrease of the flagella regulon transcripts which are needed for the synthesis and operation of the flagellum.[http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2958.2009.06939.x/full (1)] The synthesis and assembly of flagella are regulated by the transcriptional cascade composed of three levels of gene products (class I, -II and –III). Class I genes consist of a single operon encoding the proteins FlhD and FlhC that form a multimeric (FlhD4C2) transcriptional activation complex. This ‘master regulator’ stimulates transcription by binding upstream of Class II promoters. Class II genes encode proteins that assemble to form the basal body and hook of the flagellum, as well as the fliA gene that encodes the alternative σ factor σ28, also called σF. σ28 binds to RNA polymerase (RNAP) core enzyme and directs it to Class III promoters. Class III genes encode the rest of the structural genes of the flagellum, including fliC encoding flagellin, as well as the chemotaxis apparatus. [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2958.2009.06939.x/full (1)]
It has been shown that overexpression of the FlhDC operon restores motility in mutants that have been made immotile [http://jb.asm.org/cgi/content/short/181/24/7500 (2)]. Also, overexpression of FlhDC in the E. coli K12 strain MG1655 made the cells hypermotile.[http://iai.asm.org/cgi/content/abstract/75/7/3315 (3)]

Our system
To maximize the microflow system's effectivity, we want to increase the force each single bacterium can generate. Since the main motor for the flow is the flagellum we will have to modify this factor for increasing the force. Flagella in E. coli rotate at a maximum speed around 6000 rpm, which can not easily be exceeded. So if we wanted to increse the generated force we would have to opt for more flagella on the surface of our bacteria, instead of faster flagella. This is called hyperflagellation, which is a process that is not all too well studied in normally-flagellated E. coli, so we will have to test it.
The way we are hoping to achieve the hyperflagellation is by upregulating the FlhDC operon. Since FlhDC is sitting on top of the regulating cascade we want to overexpress it, so that we will get an increase in flagellar count.
The way we are going to achieve that is by isolating the operon from an E. coli and inserting the coding sequence into a BioBrick where we can control both the ribosome binding site and the type of promoter. We are going to use a constitutive promoter, so that flagella will be constantly expressed. The effects of this on other areas of the organism like the cell cycle are not entirely clear, but we will first try to overxpress the operon and then analyse the effect on the cells behavior.

Biobrick design

FlhD,C coding sequence: BBa_K343000 (Sandboxed)
FlhDC composite part: BBa_K343004

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