Team:EPF Lausanne/Interlab
What are we talking about?
In science, and in the field of synthetic biology in particular, characterizing new devices turns out to be as important as conceiving them. This not only provides a “how to use” guide for future users of your part but also allows the discovery of biologically relevant information about how it functions. When it comes to iGEM, the importance of characterization reaches huge proportion since thousands of new parts are registered each year. As a matter of fact, last year the competition launched its first InterLab Study, inviting every participating team to collaborate to measure previously existing devices. In addition to providing robust and statistically useful data, the InterLab Study aims at assessing how those measurements vary between labs. How similar are data from two teams using the same protocol? How well are the ratios conserved using two different measurement equipment? This year, these questions will be answered for the three constructs each team was given. They each contained a promoter from the widely used Anderson promoter collection that controlled the expression of a GFP. Each construct is described below. We contributed this year by measuring the three constructs in biological triplicates with a flow cytometer, which allowed us to assess the cell-to-cell variability of our samples. As part of the extra-credit assignment, we also provided technical triplicates of our data, thus determining the precision of the measurements.
Tested constructs
BBa_J23101
+
BBa_I13504
in
pSB1C3
Sequencing can be found here
BBa_J23101
+
BBa_I13504
in
pSB1C3
Sequencing can be found here
How did we achieve this?
The construction of the three devices was achieved using the BioBrick cloning system. Plasmids pSB1C3 containing promoters J23101, J23106 or J23117 were opened using SpeI and PstI (NEB) enzymes while plasmid pSB1A2 containing I13504 was digested with XbaI and PstI (NEB). pSB1C3-J23101, pSB1C3-J23106 and pSB1C3-J23117 were dephosphorylated with Antarctic phosphatase in order to prevent their self-ligation. I13504 was finally ligated with each of the open promoter-containing pSB1C3 plasmids using T4 ligase (Invitrogen). Constructs were run on a 1.2% agarose gel, purified following the QIAquick Gel Extraction Kit (Qiagen) and transformed in DH5alpha high-efficient bacteria (NEB) and plated on chloramphenicol LB agar plates. Three colonies per plate were cultured overnight in 5mL LB medium with chloramphenicol in test tubes. Cultures were spun down and pellets resuspended in 1mL PBS. Samples were measured by Accuri c6 Flow-Cytometer (BD) and data were acquired three times in arbitrary units.
More information are available in our InterLab Protocol, InterLab Worksheet and in our
Protocol
section.
What did we get?
We plotted the constructs’ mean of fluorescence across the three measurements of the three biological replicates (Fig.1). As arbitrary units were used, we also expressed the ratio between the three promoters (Fig.2) which gives a more relevant information about difference in GFP expression. We compared our results with the measured strength of those promoters from the Anderson collection . While the J23101/J23106 ratio is quite similar to the one measured by Anderson himself (only 1.67 fold difference), J23101/J23117 and J23106/J23117 ratios vary from 27 and 16 fold between our results and Anderson’s respectively. A plausible explanation for such discrepancy would be the variation between measurement instruments. While we used a Flow-Cytometer that allows finer measurements of week constructs such as J23117, it is likely that Anderson used a plate reader or any other instrument. Also, the different chassis or protocol used to prepare samples could also impact on GFP expression. Those differences are precisely what the InterLab Study intends to shed light on and we are curious to see the results other iGEM teams will obtain.
