Summary
On the fundament that RNA scaffold in vivo has been achieved, we aimed to design and assemble controllable scaffolds. A designed theophylline aptamer was added on the original scaffold D0 in order to produce an interaction with MS2 aptamer in the absence of theophylline, thus disturbing the bind of MS2 aptamer and corresponding protein. However in the presence of theophylline, the interaction would disappear with the change of theophylline aptamer. We called these controllable scaffolds 'clovers'. Three versions of clover were designed, which have different interaction sites and different relative positions between the theophylline and MS2 aptamers.
As a preliminary experiment, we use theophylline of different concentrations to test two existing parts, which consist of the gene of a theophylline aptamer and a fluorescent protein.
Clover version two has been synthesized; it was co-transformated into E.coli with Fa-MS2 and Fb-PP7 fusion proteins. As a result, in a certain range of theophylline concentration, the fluorescent intensity has a positive correlation with it, which means controllable scaffold is achieved. Besides, to our surprise, the fluorescent could reach a higher intensity than the original scaffold D0, which means our clover, makes lovers closer!
More future work will be done in the next period. We are going to synthesize clover version one and version three, which seem to have a better control effect. A comparison will be made to find the best design principle of controllable RNA scaffold. We believe that more RNA scaffolds with higher efficiency and better regulative effect will be designed and applied.
Design
We thought to add another aptamer onto the scaffold and construct an interaction between it and the MS2 aptamer, such that it could disrupt the binding of MS2 protein and the MS2 aptamer.
We thought about the well-known theophylline aptamer. The aptamer is a single RNA hairpin that binds theophylline in an inner loop region with high affinity. Previous studies have shown mutations in the loop region were tolerated as long as the loop structure was preserved. This allowed us to mutate the loop of the theophylline aptamer to create an interaction between the theophylline aptamer and the MS2 aptamer. The interaction inhibits the binding function of MS2 aptamer in the absence of theophylline. However, when theophylline is added, the fold of the loop is changed and thus the interaction will disappear, leading to the binding of MS2 aptamer and corresponding protein.
Fig.1 The control mechanism of the theophylline aptamer.
Since the reformed scaffolds consist of three aptamers, just like clovers, we call them 'clover'.
Fig.2 Our designed scaffolds are named 'clover'.
Three versions of 'clover' were designed.
Fig.3 Three version of clovers. Version one and version two have adjacent MS2 and theophylline aptamer, while vesion three has separated ones. Version one has an interaction between the loop of theophylline aptamer and the loop of MS2 aptamer, while version two and version three have an interaction between the loop of theophylline aptamer and the stem of MS2 aptamer.
Original scaffold D0:
The base sequence of original scaffold D0:
GGGAGGACTCCCACAGTCACTGGGGAGTCCTCGAATACGAGCTGGGCACAGAAGATATGGCTTCGTGCCCAGGAAGTGTTCGCACTTCTCTCGTATTCGATTCCC
Fig.4 The secondary (left) and the tertiary(right) structure of D0.
Clover version 1
The interaction is between the loop of theophylline aptamer and the loop of the MS2 aptamer.
And the theophylline aptamer is just beside the MS2 apatamer.
The base sequence of clover version 1:
GGGGUCCUCGGUGAUACCAGCAUagugacuAUGCCCUUGGCAGCACCGAGGAGGACTCCCACagtcactGGGGAGTCCTCGAATACGAGCTGGGCACAGAAGATATGGCTTCGTGCCCAGGAAGTGTTCGCACTTCTCTCGTATTCGCCCC
Fig.5 The secondary (left) and the tertiary (right) structure of clover version 1.
Clover version 2
The interaction is between the loop of the theophylline aptamer and the stem of the MS2 apatamer. And the theophylline aptamer is just beside the MS2 apatamer.
The base sequence of clover version 2:
GGGGUCCUCGGUGAUACCAGCugacuguggCCCUUGGCAGCACCGAGGAGGACTCccacagtcaCTGGGGAGTCCTCGAATACGAGCTGGGCACAGAAGATATGGCTTCGTGCCCAGGAAGTGTTCGCACTTCTCTCGTATTCGCCCC
Fig.6 The secondary (left) and the tertiary (right) structure of clover version 2.
Clover version 3
The interaction is between the loop of the theophylline aptamer and the stem of the MS2 apatamer. Although the theophylline and the MS2 apatamer is separated by the PP7 aptamer in the base sequence, they are closed according to the three- dimensional structure prediction.
The base sequence of clover version 3:
GGGGUCCUCGGUGAUACCAGCugacuguggCCCUUGGCAGCACCGAGGACUGGGCACAGAAGAUAUGGCUUCGUGCCCAGUCGAAUACGAGGAAGUGUUCGCACUUCACCUGGGACUCccacagucaCUGGGGAGUCCCAGGUUCUCGUAUUCGCCCC
Fig.7 The secondary (left) and the tertiary (right) structure of clover version 3. Although the theophyline and MS2 aptamers are separated as the secondary structure showed, in the tertiary structure, the theophyline aptamer obviously fold towards the MS2 aptamer.
Fig.8 A contrast between clover version 3 and a scaffold including a theophyline aptamer without a complementary site with MS2 aptamer. It can be easily noticed that in clover version 3, the theophyline aptamer obviously fold towards the MS2 aptamer, which indicates the interaction between the complementary sites in the theophyline and MS2 aptamers. In contrast, the scaffold without complementary sites in the two aptamers shows no approach of the theophyline aptamer to the MS2 aptamer.
Preparation: Characterize parts in registry
To make tentative steps in order to fix the theophylline concentration of our clover coexpression experiment, we test some theophylline riboswitches tagged with fluorescent proteins.
BioBrick Part K537009:
To characterise the theophylline riboswitches (part K537009, iGEM11_WITS_CSIR_SA), we quantified their activation at different theophylline concentrations (0 mM, 1 mM, 5 mM, 10 mM and 20 mM) over 2 hours using fluorometry.
Competent E. coli (strain DH5a) cells were transformed with plasmid vectors containing the "Promoter-Theophylline riboswitch -Venus-Double terminator". The bacterial colony appeal pink.
Cultured until the mid-log phase of growth, a different concentration of theophylline was added to each culture for induction. The activation of the riboswitch was detected as a fluorescent response as a result of increased translation of the fluorescent protein Venus, in the presence of the activator. Before fluorescence assay, we wash the culture with PBS.
A Synergy hybrid reader was used to excite the cultures at 505 nm and the intensity of the emission was detected at 535 nm. Empty bacteria were used to correct for auto fluorescence (IGEM11_WITS_CSIR_SA offered exciting at 514nm and emission at 528nm, but 514&528 is too close for our machine to detect.)
We have two end points of the OD 630 of each sample.
"Fluorescence intensity / OD" increases greatly with theophylline concentration.
Fig.9 The 5 different concentration of theophylline comparision on part K537009 theophylline robswitch tagged with venus YFP. Excitation at 505nm and emission at 535nm. Up to 20mM theophylline, cells didn't show side effects and YFP production is proportioned with theophylline concentration, showing that K537009 is an effective riboswitch which can be regulated by theophylline.
Fluorescence Microscope could also show this part work beautifully.
Except for the difference that though K537009 is an YFP, we excite it at 532nm (green light) and it glow red.
Fig.10 The 5 different concentration of theophylline comparision on part K537009 theophylline riboswitch tagged with venus YFP. The brightfield (BF) images in the right column depict all bacterial cells. The venus images in the left column depict bacterial cells which emitted fluorescence. We excite it at 532nm (green light) and it glow red, seeing the obvious trend that when adding more theophylline, cells showing Veuns appeal more.
BioBrick Part K411003:
This is a"pLAC promotor,Theophylline-inducible Riboswitch, GFP+Terminator" part made by 2010 NYMU-Taipe. We make similar tests as K537009 at different theophylline concentrations (0 mM, 0.1mM, 0.3mM, 0.5 mM,1mM,5mM,10mM and 20mM) over 2 hours using fluorometry.
Synergy hybrid reader detects effective effects of theophylline on GFP production.
Fig.11 The 8 different concentration of theophylline comparision on part K411003 theophylline robswitch tagged with GFP. Excitation at 480nm and emission at 535nm. Up to 10mM theophylline, cells didn't show obvious side effects and GFP production is proportioned with theophylline concentration, showing that K411003 is an effective riboswitch which can be regulated by theophylline. When theophylline concentration is beyond a certain degree (about 10 mM), it somewhat affect cell growth and GFP production.
Through Fig9 and Fig11 we find that when theophylline concentration scale is 0-1mM (especially 0-0.5mM); the response of fluorescence protein to theophylline is more significant with bigger slope. So we decided to carry out our clover 2 characterization with theophylline concentration scale 0-1mM.
Characterization
We characterize clover 2(Part K738002) we designed in split GFP methods.
Clover 2 is synthesized by Genscript and is a regulatory and tunable RNA scaffold with a theophylline aptamer. As we design it, when theophylline is added, MS2 aptamer will be free to combine FA tagged MS2 protein and closer with PP7 aptamer thus give out increasing light. We use fluorescence microscope, confocal microscopy and microplate reader to test our riboscaffold clover 2. For microplate hybrid reader experiments, fluorescence intensity/ OD 630 are used to be measurement criteria for comparison of the ability of give out light in microplate hybrid reader test.
Scaffold
We use fluorescence complementation to detect 3D comformational change of our riboscaffold clover 2. Green fluorescent protein (GFP) split into two halves (FA and FB) fused to the PP7 or MS2 aptamer binding proteins was used. Cells expressing FA and FB alone or clover 2 without the split GFPs displayed little fluorescence. However, the coexpression of clover 2 with the split GFPs showed increased fluorescence. Thus, our RNA scaffold clover 2 served as docking sites to promote protein-protein interactions in cells.
We make a series of cotransformation containing D0,clover2,FA,FB,FA+D0,FB+D0,FB+clover 2,FA+FB,and FA+FB+D0 & FA+FB+clover2. At mid-log phase we low-temperature-induced the E.colis by adding IPTG (0.2mM & 1mM) at 20 degree. After 5 hours, strains containing clover 2 were adding 0.5mM Theophylline. Successfully results are shown as we expected that, strains expressing only RNA scaffolds almost don't give out light, strains expressing FA+FB give out medium light and strains containing RNA scaffold &FA+FB emit a powerful beam after excitation.
To our surprise, the RNA scaffold clover 2 serves as a more effective scaffold than the origin D0, whose luminescence efficiency is 124.67% and performs better than D0 by 39.5%.
(Formula: `luminescence \quad efficiency=\frac{\frac{FI}{OD(FA+FB+scaffold)-\frac{FI}{OD(FA+FB)}}}{\frac{FI}{OD(FA+FB)}}`
Regulate and control by Theophylline
To characterise the theophylline tuned RNA scaffold clover 2 (part K738002), we quantified their activation at different theophylline concentrations (0 mM,0.1mM, 0.2 mM, 0.3mM, 0.4 mM, 0.5 mM, 0.6 mM, 0.7 mM,1 mM) over a period of time using fluorometry. Competent E. coli (strain BL21*DE3) cells were transformed with plasmid vectors containing the riboscaffold and were cultured until the mid-log phase of growth, then 0.2mM IPTG were added. After 3 hours, a different concentration of theophylline was added to each culture for induction. 2 hours later, we use PBS to wash the culture and prepare for following tests. The regulatory effect of the Theophylline aptamer was detected as a fluorescent response as a result of increased release of the MS2 aptamer. MS2 aptamer will be free to combine MS2 protein tagged FA and be closer with PP7 aptamer when Theophylline adding. A Synergy Hybrid Reader was used to excite the cultures at 480 nm and the intensity of the emission peak was detected at 535 nm. 3 replications were taken each culture, and 4 repeated experiment.
We find that up to a certain point (0.5 mM), a positive correlation exists between the GFP production and theophylline concentration, indicating that at first MS2 aptamer and theophylline aptamer are interacting(through specific base pairing) and close, when theophylline goes in, MS2 aptamer combines FA+MS2 and interact with FB+PP7 on PP7 aptamer thus give out fluorescence light. More theophylline, more light. SAS software results show significant variation(P-value<0.05) between 0mM theophylline and 0.5mM theophylline (optimum theophylline concentration to make clover 2 work) effects on clover 2.
It turns out that our riboscaffold clover 2 can be regulated and controlled through conformational change by theophylline. This scaffold, by theophylline management, could have a variety of functions, more than accelerate the reaction, but whether to accelerate or not, the degree of acceleration and even reduce the reaction rate.
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