After successfully building the plasmids that contain the genes for methanol conversion, our next ambition was to characterize the strains with different approaches.
Don't forget to take a look into the results of our 13C labelling experiment!
- The expression tests of mdh, hps, phi and xpk can be found under Single Expression. We expressed all enzymes separately to show that the heterologe expression of the coding sequences is possible in E. coli.
- The performance of several E. coli strains that contain our constructs under varying conditions is documented as Physiology. There are all results of shake flasks and deepwell microtiterplate experiments summed up.
Very detailed information can be found additionally under Documentation in our Notebook.
Single Expression
To check the expression of mdh, hps, phi and xpk, we inserted them into a vector downstream a strong lacI repressible T7 promoter. Therefore, we used the [http://parts.igem.org/wiki/index.php?title=Part:BBa_K1362091 pSB1A30] plasmid that was built by Team Heidelberg 2014[1]. In this vector, you find the BioBrick scar downstream the promoter.
All four genes were cut with EcoRI and PstI and ligated into a pSB1A30 plasmid that was digested with the same enzymes. As these construct require a T7 polymerase, they were transformed into BL21 Gold (DE3). Thus, we created microorganisms that, after IPTG induction, expressed the four MCC genes separately. For testing, we ran SDS-PAGEs with cell pellets that should contain the enzymes.
| BioBrick |
enzyme |
molecular weight [kDA]
|
| BBa_K1585210 |
Mdh |
40.70
|
| BBa_K1585211 |
Hps |
22.60
|
| BBa_K1585212 |
Phi |
19.94
|
| BBa_K1585213 |
Xpk |
92.63
|
Results of Single Expression
After successfully building the plasmids, we tested the expression of the four genes separately. The SDS-PAGEs show that each of the enzymes is present in the cell after IPTG induction.
As a negative control, we used a BL21 Gold (DE3) strain that expresses RFP.
Laboratory Notebook of Single Expression
Mdh Characterization
The expression of the Mdh was confirmed via a SDS-PAGE showing clearly visible bands at the expected weight of 40.7 kDa. In parallel to whole cells, supernatant of lysed cells and the respective cell fragments were checked via SDS-PAGE. The cell fragments showed the strongest bands indicating the formation of inclusion bodies containing Mdh when expressed under the control of a T7 promoter. To avoid this, we transformed different strains of E. coli (SHuffle T7 Express and [http://www.lucigen.com/OverExpress-C41-DE3-and-C43-DE3-Competent-Cells/ C43]) as alternatives to BL21 Gold (DE3) which are known for more efficient protein expression and also tested lower cultivation temperatures. Additionally, it was confirmed that all strains are able to grow on M9 medium because we made the experience that not each strain is able to grow on M9.
However, the variation of strains and cultivation temperatures showed no significant effect on the Mdh becoming incorporated into inclusion bodies.
To test the functionality of the expressed Mdh we used a modified version[2] of the colorimetric and fluorometric formaldehyde assay described by Nash[3]. The whole-cell samples of BL21 Gold (DE3) showed the highest formation of formaldehyde proving the strongest activity of the Mdh compared to the other strains. Neither in the respective cell fragments nor in the supernatant of the lysed cells a similarly high activity could be observed. This indicates that the Mdh functions best in its natural environment within the cell and the cell membrane possibly representing a barrier protecting the Mdh from the harsh assay environment.
Achievements
- Proving the catalytical activity of the Bacillus methanolicus methanol dehydrogenase 2 in E. coli via Nash Assay despite the formation of inclusion bodies
Expression verification & localization of the Mdh
The expression of the Mdh was verified by doing an SDS-PAGE. To localize the Mdh not only whole cells were used but also the fragments of lysed cells and the respective supernatant. The expected bands were clearly visible in all samples proving the expression of the Mdh. However, the Mdh specific band had the highest intensity in the sample of the cell fragments whereas the same band was only of slight intensity in the supernatant. This indicates that the Mdh is incorporated into inclusion bodies.
To avoid the incorporation of the Mdh into inclusion bodies, the alternative E. coli strains SHuffle T7 Express and C43 were used. SHuffle T7 Express allows more efficient protein folding in the cytoplasm and lacks proteases whereas C43 allows the expression of toxic proteins. Additionally, cultivation at a lower temperature of 30 °C was tested. It was shown that all strains were able to grow on M9 medium, however, another test for the expression and localization of the Mdh did not reveal major differences between the strains. In every case the Mdh was still incorporated into inclusion bodies.
Functionality of the expressed Mdh
To test if the expressed Mdh is functional, we modified a colorimetric and fluorescent formaldehyde assay described by T. Nash. In the presence of formaldehyde the yellow and fluorescing diacetyl-dihydro lutidine is formed. In the reaction catalyzed by the Mdh methanol is converted to formaldehyde which can be detected by the mentioned assay.
First, the different cells as well as their respective fragments and lysate supernatants were screened for formaldehyde production. The samples were taken 6 h after induction from shake flask cultures. Several cells, fragments and supernatants showed a production of formaldehyde but the Mdh activity in the intact cells was higher for each strain.
The formaldehyde assay was repeated only with whole cells and additional samples taken 20 h after induction. Moreover, the assay was conducted not only at 37 °C but also at 30 °C. The GlgC expressing BL21 Gold was used as a negative control. Again, in several strains functional Mdh could be detected and some general conclusions could be drawn:
- More formaldehyde was produced in the samples taken 6 h after induction
- Strains cultivated at 37 °C show a stronger response than the same ones cultivated at 30 °C
- The assay works better and faster at an incubation temperature of 37 °C
By far the highest formaldehyde production was observed in the BL21 Gold (DE3) cells 6 h after induction cultivated at 37 °C despite its formation of inclusion bodies.
To show the significance of the production of formaldehyde the assay was conducted with the Mdh expressing BL21 Gold (DE3) cultivated in the labled methanol experiment along with the BL21 Gold (DE3) expressing GlgC as a negative control, both in multiple replicates.
The Mdh expressing strain showed significantly more formaldehyde production indicating a functional expression of the Mdh.
Conclusion & Discussion
The functional Mdh was successfully expressed in multiple strains. The strongest response in the formaldehyde assay was reached in BL21 Gold (DE3) cultivated at 37 °C in M9 medium. Although inclusion bodies containing the Mdh are formed in BL21 Gold, a high activity in the intact cells could be observed. This suggests that either a sufficient amount of Mdh still remains functional and in solution or the enzymes on the surface of the inclusion bodies still have a catalytic activity. The fact that a higher activity is reached in whole cell samples compared to the cell fragments and the lysate supernatant might be due to the protective property of the intact cell membrane shielding the enzyme from the harsh assay conditions and keeping it in its native environment.
Physiology
After successfully building strains that genetically meet all conditions to metabolize methanol, we wanted to examine the growth performance of a strain that carries the engineered constructs.
The strain with the polycistronic plasmid behind J23119 promoter and the one expressing the Mdh in [http://parts.igem.org/wiki/index.php?title=Part:BBa_K1362091 pSB1A30] were cultivated on different media with varying concentrations of methanol. Our growth experiments were performed in shake flasks and Biolectors [4].
To measure the OD we used, among other things, the automated cell density monitoring devices of [http://www.aquila-biolabs.de Aquila Biolabs].
First, we wanted to know how the amount of methanol affects the growth of E. coli in general. Therefore, we cultivated a BL21 Gold (DE3) strain with an empty pSB1KRDP backbone at different methanol concentrations on M9 medium. As expected, high concentrations of methanol in the medium clearly decelerate the growth of the organisms.
Next, we wanted to test the overall growth performance of our polycistronic plasmid in BL21 Gold (DE3). In this experiment, a strain with the polycistronic construct, the Mdh expression plasmid and a control were grown on M9 medium without any modifications. The best growth could be detected for the strain with the four genes in a polycistronic frame. This findings surprised us because the burden of expressing the four genes should inhibit growth in comparison to strains with less additional overexpressed genes. In later attempts, this results could not be reproduced and the strain with the polycistronic version of the methanol conversion genes grew worst compared to the others.
Subsequently, we were able to show that the lag phase of a strain with our polycistronic construct is much shorter compared to a strain with the Mdh expression plasmid and a control with an empty pSB1KRDP backbone when it is grown on M9 medium with 1.6 M methanol. This amount of methanol equals 6.48 percent per volume of the fermentation broth. The polycistronic strain is the only one that grows at all on on this methanol concentration that actually is the highest concentration where we ever observed any growth.
Even though strains with our polycistronic plasmid grow best on high methanol concentrations compared to the others, methanol in the medium still affects the growth of it a lot. The strain grows to the highest densities in the shortest time, when there is no methanol at all in the medium. The lowest growth rate was detected when it was grown on 1.6 M methanol.
Nevertheless, it is still remarkable that this strain has an exponential phase at all at these methanol concentrations.
Unfortunately, these results were not reproducible. In later experiments, there were also results where the control grew faster than the strain with our methanol conversion genes on high methanol concentration.
As the Mdh represents the bottleneck of the whole pathway we tested its activity in the strains with the polycistronic plasmid. For this the colorimetric and fluorescent assay to detect formaldehyde first described by Nash was done these strains. Whole cells, cell fragments and lysate supernatants were tested for the polycistronic strains. The highest formation of formaldehyde could be observed in the assay using whole cells.
It was proven that the Mdh is expressed functionally in the strains containing the polycistronic construct. However, if compared to the activity of the strain only expressing the Mdh it is rather low.
Achievements
- we measured the toxicity of methanol on E. coli BL21 Gold (DE3) without any genetic modification by calculation the growthrate.
Result
- We showed once but not reproducible that the strain with our polycistronic construct had better growth properties with increasing methanol concentration compared to control and Mdh expression strain The end OD value is nevertheless under the values of a strain with an empty backbone.
- All failed experiments during the developement of our functional assay are documented here. The successful Mdh characterization was done with an alternative Nash Assay. The in vitro assay [5] of Mdh was not reproducible for us with our remained time.
References
- ↑ https://2014.igem.org/Team:Heidelberg/Parts#Backbones
- ↑ Kleeberg & Klinger. 1982. Sensitive formaldehyde determination with Nash's reagent and a tryptophan reaction. Journal of Pharmacological Methods, 8(1), 19-31.
- ↑ Nash. 1953. The colorimetric estimation of formaldehyde by means of the Hantzsch reaction. Biochemical Journal, 55(3), 416-421.
- ↑ http://www.m2p-labs.com/microbioreactor-biolector-gbl100
- ↑ Müller JE, Meyer F, Litsanov B, Kiefer P, Potthoff E, Heux S, Quax WJ, Wendisch VF, Brautaset T, Portais JC, Vorholt JA. Engineering Escherichia coli for methanol conversion. Metab Eng. 2015 Mar;28:190-201. doi:10.1016/j.ymben.2014.12.008. Epub 2015 Jan 14. PubMed PMID: 25596507