Composite

Part:BBa_K2240000

Designed by: iGEM2017_HKUST   Group: iGEM17_Hong_Kong_HKUST   (2017-10-18)
Revision as of 18:15, 31 October 2017 by Karencheng (Talk | contribs)


AHL sensor with positive feedback loop, GFP output and antisense RNA type 1 inhibition

This part can detect the deliberately released stimulus so as to initiate the process of 'knockout'. Considering that the released stimulus can be diluted in an environment, a positive feedback loop is introduced to amplify the signal. 3OC6HSL, which is a member of acyl-homoserine lactone (AHL) family, would be the inducer. 3OC6HSL originated from V. fischeri is a lipid molecule that can diffuse through bacterial cell membrane to facilitate the cell-to-cell communication.

This part begins with the TetR repressible promoter (BBa_R0040), which can act as a constitutive promoter of the downstream protein - LuxR (BBa_C0062) under the absence of repressor TetR. Once the 3OC6HSL is added, LuxR will form a complex with 3OC6HSL and then activates the downstream promoter, PluxR (BBa_R0062). In the end, the production of the LuxR can be boost, allowing the accumulation the LuxR.

After the activation of PluxR, LuxI protein (BBa_C0061), which is an autoinducer synthetase catalyzes 3OC6HSL from S-adenosyl-L-methionine (SAM) in cell. Due to the increase of the 3OC6HSL/LuxR complex, the entire part starting from PtetR to LuxI generates positive feedback loop, hence further induces the PluxR. Apart from this, the 3OC6HSL molecule can also diffuse to the extracellular environment and induce the cells nearby.

Owing to the positive feedback loop, this part would start contributing to signal emission whenever it receives 3OC6HSL molecules. As a result, it is expected to elevate the efficiency of activation under induction.

Considering the difficulties that previous iGEM team encountered - the leakiness of PluxR (See experience in BBa_F2620), we tried to get rid of this by adding antisense RNA Binding regions and antisense RNA (asRNA), interaction of which, could counteract the activity of basal level. Ideally, this could help tackling the thorny issue via degrading the mRNA of the downstream proteins.


Usage & Biology

The antisense RNA (asRNA) we used had two important characteristics which might help boost the efficiency in reducing leakiness: possessing the sequence complementary to the of mRNA of ABR, and a Hfq (RNA binding protein) binding site. The affiliation of asRNA to the complementary ABR would prevent ribosome from binding to the mRNA of the targeted RBS. Meanwhile, the presence of a Hfq binding site would help reduce the translation of the 3OC6HSL/LuxR complex since Hfq binding site was suspected to recruit RNase to degrade the targeted RNA chain. With these two effects, the leakiness of PluxR could be lowered.

There are two asRNA binding regions (ABR) in total. One is placed right before the targeted ribosomal binding site (RBS), which is upstream of the LuxI (BBa_C0061) and GFP (BBa_E0040) while another one is placed downstream of the positive feedback loop.

Under the presence of the 3OC6HSL, 3OC6HSL/LuxR complex could repress PluxL, reducing the production of the asRNA, increasing the production of mRNA of LuxI. This meant that the maximum level of LuxI translation could be reached after 3OC6HSL induction.

Based on the above reasons, it is expected that the ability in sensing the overall population would become more sensitive due to the positive feedback loop, whereas the leakiness could be reduced without the presence of inducer.


Antisense RNA Type 1 & 2

To provide an alternative, two antisense RNA sequences (type 1 and type 2) with a few base pairs different were designed for degrading the mRNA. Both of them were obtained from the paper, Development of design rules for reliable asRNA behavior in E. coli, and then slightly modified. Being incorporated into an inducible system with positive feedback loop (PFB) that was able to produce GFP expression, they could be evaluated for their efficiency in reducing the basal level via measuring the GFP output with the presence of AHL.

Results
Fig.1 Error bar presents SD from 6 biological replicates.
Fig.2 Error bar presents SD from 6 biological replicates.



1. Antisense RNA (asRNA) can reduce basal expression level

The efficiency of basal level reduction for asRNA type I and II were compared with the control (w/PFB) prior to AHL induction ([AHL]=0M) and after the induction ([AHL]=1.00E-05 M). Unpaired t-test analysis suggested that their differences could be considered as highly statistically significant.

2. Antisense RNA type II reduces basal level more significantly

Based on the results obtained from the unpaired t-test calculation, there were significant differences between antisense RNA type I and type II under both cases - before and after AHL induction (1.00E-05 M). Fig. 1 and 2 implied that construct with asRNA type II could reduce basal expression level more significant than asRNA type I. This may be due to higher GC content of its complementary sites (55% for asRNA type II comparing to 47.4% for asRNA type I).



Signal Amplification

GFP was measured in order to examine the efficiency for the positive feedback loop to amplify the signals with the presence of the antisense RNA.

Results


Sequence and Features


Assembly Compatibility:
  • 10
    COMPATIBLE WITH RFC[10]
  • 12
    COMPATIBLE WITH RFC[12]
  • 21
    INCOMPATIBLE WITH RFC[21]
    Illegal BglII site found at 1751
  • 23
    COMPATIBLE WITH RFC[23]
  • 25
    COMPATIBLE WITH RFC[25]
  • 1000
    INCOMPATIBLE WITH RFC[1000]
    Illegal BsaI.rc site found at 1004
    Illegal BsaI.rc site found at 2455
    Illegal BsaI.rc site found at 2674


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