Theory

As already mentioned in the project section we decided to work with the Calvin-cycle. There are different reasons for this. One the one side we searched for a method to cultivate aerobically and on the other side we searched for a cycling system which is not given in every carbon dioxide fixation possibility.
In E.coli are three enzymes missing to enable the whole cycle. Our goal is to transform the missing genes from different organisms. Our main research is based on a publication by Bonacci et al., 2011. The idea was to use the carboxysom as a microcompartiment for E.coli from Halothiobacillus neapolitanus for an efficient RuBisCO activity. The problem of the RuBisCO is the affinity to oxygen which lead to non fixating cycle. A carboxysom establishs a higher concentration of carbon dioxide in the microcompartiment. The substrate for the RuBisCO is provided by an enzyme called phosphoribulokinase which took from Synechococcus elongatus. The last missing enzyme was taken from Bacillus methanolicus and is called sedoheptulose 1,7-bisphosphatase (Stolzenberger et al., 2013).
A problem for our system would be the glycolysis which we aimed to inhibit with a knock-down of the phosphofructo kinase. We called this system the gluco-switch.
The main goal for characterization was to establish a new enzyme purification system based on an intein tag with chitin binding domain. It could be used by integrating a coding sequence into the vector.

Phosphoribulokinase

Parikh et al., 2006

Toxicity of phosphoribulokinase without RuBisCO

Sedoheptulose 1,7-bisphosphatase

For the characterization of the sedoheptulose 1,7-bisphosphatase (SBPase / glpX) we did an enzyme assay with a His-Tag purification as described before (Stolzenberger et al., 2013).
The proteins were overexpressed by adding 1 mM IPTG for the T7 promotor. The increasing amount of protein could be verified through a SDS-PAGE..

Proteinexpression of fba

Proteinexpression of tkt

Proteinexpression of glpX

All three SDS-Gels showed a clear band like described in Stolzenberger et al., 2013. We purified the transketolase (tkt) and the fructose bisphosphate aldolase (fba) as well as the sedoheptulose 1,7-bisphosphatase with the His-Tag mediated purification system.

Protein purification of fba

Protein purification of tkt

Protein purification of glpX

For the purified enzymes we performed a Bradford assay.

The Bradford assay showed high concentrations of Tkt and Fba as well as a very low concentration of GlpX. After the purification we performed an enzyme assay as shown below.

SBPase assay

The product of the reaction, sedoheptulose 7-phosphate, could be identified via HPLC. We made different approaches to characterize all reactions.
Reaction mix:

• 20 mM Fructose 6-phosphate
• 20 mM Glyceraldehyde 3-phosphate
• 20 mM Dihydroxyacetonephosphate
• 10 µM Thiamine pyrophosphate
• 2 mM Manganese chloride
• 50 mM Tris-HCl

In the first approach we add no enzyme to verify that no product is generated. The second approach includes the transketolase which does the reaction of F6P and GAP to erythrose 4-phosphate and fructose 1,6-bisphosphate. In the third approach fructose bisphosphate aldolase was added which converts erythrose 4-phosphate with dihydroacetonephosphate to sedoheptulose 1,7-bisphosphate. In the last approach the sedoheptulose 1,7-bisphosphatase (glpX) was added which results in sedoheptulose 7-phosphate. All intermediates could be verified in all approaches as expected. This measurement showed the activity of the SBPase in vitro. We did a comparison between 37°C and 50°C. The transketolase and aldolase performed with a higher activity at 37°C which resulted in more products. The sedoheptulose 1,7-bisphosphatase acitvity is higher at 50°C but also shows activity at 37°C. This result means that our approach is able to enable the whole Calvin-cycle with glpX.
We decided to use glpX as a target for amplification and transformation because of the shown acitivity. The finished construct of ptac_glpX in pSB1C3 was cultivated in M9 glucose in comparison to the wildtype. We performed two biological replicates and two technical replicates.

Cultivation experiment in M9 glucose

The cultivation shows that the modified strain has a longer lag phase before the entrance in exponential phase. Two hours after IPTG induction of the strain results in a decrease of growth in comparison to the uninduced strain. There are two possible explanations for this behaviour. One the one side IPTG acts as a poison for bacteria which may result in growth decrase and on the other side the production of the protein can result in decreased grwoth. We exclude IPTG as a reason because earlier cultivations showed that 1 mM IPTG have no effects on the wildtype growth. Entering the stationary phase takes place one hour before the induced strain. The induced strain also shows a higher OD. By inducing the SBPase in E. coli the substances for glycolysis are deflected towards other pathways. These reactions are reversible which means that the glucose of the M9 medium is not metabolized in another pathway. This time shifted use of glucose results in a higher OD in two biological replicates.
This result shows that the SBPase does not limit the growth maximum of E. coli.

Gluco-Switch

Protein purification

For the purpose of characterizing our BioBricks we thought of using enzyme assays to verify the functionality of different proteins. Enzyme assays depend on purified enzymes. A typical purification approach is the His-Tag mediated purification system. The disadvantage of this system is that the tag remains attached at the enzyme after the purification and has to be cleaved afterwards. A further development of this system is the intein tag mediated purification.
By adding an intein tag attached to a chitin binding domain to the enzyme of interest a purification can be achieved. The chitin binding domain binds the column on which chitin beads are stored. After adding binding buffers and washing solutions an elution with DTT allows to cut the attachment of the intein tag to the coding sequence. The enzyme is eluted from the column and can be stored in the desired buffer. The chitin binding domain and intein tag can be eluted from the column afterwards to reuse the column.
We implemented this system in the pSB1C3 backbone by combining the T7 promotor with RBS and intein tag with chitin binding domain.

By designing gibson assembly primers with following overhangs it is possible to add a coding sequence between the first and the second part of the purification vector (add the gene specific part behind the overhang with the right orientation):
>GSP_fw
CTATAGGGAAAGAGGAGAAAT
>GSP_rev
CTAGTGCATCTCCCGTGATGCA

Note: The stop codon of the coding sequence has to be deleted through primer design.

It may be possible to redesign the pSB1C3 backbone to the purification vector by including the T7 and RBS as well as the intein tag with chitin binding domain into the backbone. The restriction sites for BioBrick assembly may be placed in between both patterns. This would allow an in frame addition of the coding sequence by using BioBrick assembly (Note again: The stop codon has to be deleted during the amplification of the coding sequence).
Because of problems during the transformation of the coding sequences we were not able to characterize this BioBrick.

References

• Stolzenberger et al., 2013. Characterization of Fructose 1,6-Bisphosphatase and Sedoheptulose 1,7-Bisphosphate from the Facultative Ribulose Monophosphate Cycle Methylotroph Bacillus methanolicus. Journal of Bacteriology, Vol. 195, pp. 5112-5122
• Bonacci et al., 2011. Modularity of carbon-fixing protein organelle. PNAS, vol. 109, pp. 478-483
• Parikh et al., 2006. Directed evolution of RuBisCO hypermorphs through genetic selection in engineered E.coli. Protein Engineering, Design & Selection, vol. 19, pp. 113-119