Results

Improvement of a previous part, BBa_K13372001

As a part of the characterization of a previous existing Biobrick Part, we have chosen the [http://parts.igem.org/Part:BBa_K1372001 BBa_K13372001] biobrick from the Paris-Saclay 2014 project This is not a lemon. It was designed to mimic the ripening of a lemon in E. coli by a salycilate-inducible expression of a suppressor tRNA.

The Paris Saclay 2014 team chose to use chromoproteins to express these colours in E. coli. Chromoproteins are reflective proteins that contain a pigmented prosthetic group and do not need to be excited to be seen. They fused a yellow chromoprotein with a blue one in order to display a green color. This construction is referred as the green fusion chromoprotein. In order to make the bacteria ripe like a real lemon, they decided to take advantage of the fusion protein’s design by using a translational suppression system. They added an amber codon (stop codon) within the linker separating the yellow and the blue chromoproteins genes. Therefore, the suppressor tRNA will suppress amber codon allowing the translation of the green fusion chromoprotein in presence of salicylate. Conversely, the down regulation of the suppressor tRNA in absence of salicylate will allow bacteria switch from green to yellow, thus simulating the ripening of a real lemon. This system is referred to as the colour switch system.

summary of the lemon ripening project

The tRNA used is the supD suppressor tRNA. It has been placed under control of a salicylate inducible promoter Psal. Its role is to suppress the introduced amber codon. The nahR gene encodes a transcriptional regulator that is induced by salicylate and thus binds nah or Psal promoters. In presence of high salicylate concentration in the agar media, supD will be expressed and so the green fusion chromoprotein: bacteria will display a green color. However, as bacteria grow into agar, less salicylate will remain available into the media. Thus, the decrease of the nahR-salicylate complex amount within bacteria will lead to supD downregulation through time. In turn, decrease of supD amount will result in less codon readthrough and so less translation of the green fusion protein and more translation of the yellow chromoprotein. As a result, bacteria will gradually change from green to yellow.

explanatory diagram of the lemon ripening

Characterization

In order to characterize the biobrick, the color switch system (BBa_K13372001) was tested on three different constructions:

• TAA: LacZ and Luc coding sequences in the same open reading frame, separated with an ochre stop codon.
• TQ: LacZ and Luc coding sequences in the same open reading frame.
• TAG: LacZ and Luc coding sequences in the same open reading frame, separated with an amber stop codon.

Each condition was tested under three different salicylate concentrations. In order to achieve that, both measurements of Beta-Galactosidase and Luciferase activities were performed on bacteria cultures.

The experiment was conducted on three sets of cultures of bacteria:

Remarque : attention on doit parler du plasmide contenant BBa_K13372001 + on a pas dit avant pour pcl qui s’appelle d’ailleurs pcl99

• TAA: BL21|BBa_K1372001 and pcl_TAA
• TQ: BL21|BBa_K1372001 and pcl_Tq
• TAG: BL21|BBa_K1372001 and pcl_TAG

Each of those sets of culture were incubated with three different salicylate concentrations: 0, 30µM and 1mM.

Each construction (TAA, TQ or TAG) was tested on 3 different clones (clones 1, 2 or 3), with 3 different salicylate concentrations (0, 30µM or 1mM), with in addition a negative control sample.

Pcl_TAA construction contains two TAA stop codon within the green fusion chromoprotein linker. This codon is not recognized by the supD suppressor t-RNA, thus no blue fluorescence is expected. pcl_Tq construction does not contain any stop codon, thus the green fusion chromoprotein construction should entirely been transcripted and emit both yellow and blue fluorescence. pcl_TAG contains the TAG codon recognized by supD suppressor t-RNA. Thus the transcription and the blue fluorescence of AeBlue should be inducible by salicylate. The luciferase luminescence is expected to vary depending to the different constructions conditions and to salicylate concentrations, instead of the Beta Galactosidase activity, which will remain constant. Thus luciferase data were normalized with those from Beta Galactosidase and our results are expressed as the Luciferase/Beta-Galactosidase activity. This ratio is then independent of the level of transcription, initiation or mRNA stability.

The Tq plasmid does not contain any stop codon between LacZ and Luc. Thus, no matter the salicylate concentration, both Luciferase and Beta Galactosidase activities are supposed to be detected. As expected a high level of Luciferase/bGal activity is observed, but the ratio decreases when salicylate concentration increases. Indeed, both activities of Luciferase and bGal drop from 30µM of salicylate, but luciferase activity was more affected by the salicylate than bGal one. One may hypothesis that salicylate inhibits differently both reporter proteins activities, with a stronger inhibition of luciferase activity. We cannot determine whether this inhibition is due to a physiological consequence onto bacteria metabolism or occures after protein extraction.

In order to read the next results, we calculated a readthrough percentage, by doing a ratio between luciferase/bGal from TAA or TAG constructs and luciferase/bGal from TQ. In this way, we obtain a readthrough percentage.

In TAA condition, regardless of the salicylate concentration, there is no significant Luciferase activity, so the ratio remains very low at any concentrations. We conclude that supD suppressor tRNA is very specific of the TAG codon and has no impact on the TAA stop codon.

In TAG condition we can see an increase of stop codon readthrough activity with the increase of the salicylate concentration. In comparison to the results obtained with the TAA construction, the readthrough level increases similarly to the concentration of salicylate. This indicates that TAG stop codon is efficiently readthrough in presence of supD tRNA, allowing the production of a significant amount of luciferase.

In conclusion, the Psal promoter is fully inducible by salicylate and the suppressor tRNA is functional to suppress the TAG codon. These experiments demonstrate that BBa_K1372001 is fully functional.