UNDER CONSTRUCTION

In order to build dCas9-controlled circuits, we aim to use dCas9-inducible synthetic promoters to mimic the behaviour of a transistor (Go take a look at our design (LINK) if you haven't !). To see whether such promoters could be used in wider-scale circuits, we identified and tested the following desirable properties :

• Transistor response : Is the transistor inducible? Is its output modulable allosterically
• Transistor mutability : Is it possible to mutate the dCas9-targeted sites on transistor-like promoters without altering their behaviour and output levels ? Would it be possible to make a family of homogenous transistors that can be regulated independently?
• Transistor orthogonality : is regulation of a transistor specific? Is there cross-talk? Could several transistors work simultaneously?
• Transistor chainability : Can transistors can be linked serially to allow for multiple levels of information processing ?

Transistor response: BBa_ is inducible. Activation is repressed when both activation and inhibition sites are bound by dCas9-ω

dCas9 binds DNA complementary to its guide RNA (gRNA). The location it binds to may therefore be controlled by producing specific gRNAs.

By targeting dCas9 fused to a RNAP recruiting subunit (dCas9-ω) to an adequate distance from the transcription starting site (TSS), J23117 (BBa_(LINK)) was successfully induced, which represents a PNP transistor in “on” state.

We chose to test two sites close enough to the TSS which when bound by dCas9-w are likely to sterically hinder RNA Polymerase (RNAP). By itself, targeting dCas9-w to inhibition sites (R1, R2), sometimes resulted in slight activation (repeat (LINK)) and sometimes in slight inhibition (repeat (LINK)). This might be explained by the already low promoter strength of J23117 (close to autofluorescence in our measurements and consistent with the registry(LINK)) and the presence of the activating ω subunit on dCas9-ω. Overall, R1, R2 and R1/R2 produce fluorescence close to basal levels and the transistor is considered to be in “off” state.

Yet, when activation and inhibition sites are targeted simultaneously, dCas9-w bound to inhibition position hinders induction (A/R1, A/R2). We found out that A/R2 may virtually suppress activation; the promoter is in “off” state. Such an effect has been reported for S. cerevisiae (REF) but -to our knowledge- hadn’t been tested in E. coli.

The Relative Fluoescence Units (RFU) discrepancy between A/R1 and A/R2 could be explained by our modeling simulations (LINK). A/R2 produces an output consistent with predictions for a model where activation and inhibition sites may be bound by dCas9-w without steric hindrance between the dCas9-ws: binding is independent. A/R1 on the other hand behaves as if co-binding of the activation and inhibition sites were impossible. Interestingly, we notice that for A/R1 the activation and the inhibition sites are on the same strand, whereas in A/R2 the sites are a bit farther and on different strands. If you are interested, please take a look at the promoter modeling (LINK) page for a plausible explanation.

Transistor Mutability: The promoter’s regulatory sites may be mutated while conserving its transistor behaviour

We selected dCas9-w targets outside of the -10 and -35 regions on BBa_ and mutated them. Thereby, we created an alternative transistor (BBa_) and set of gRNA inputs.

The basal levels of the original promoter and the alternative one are very similar. Nielsen (REF) has reported the use of mutated synthetic promoters between -35 and -10 regions to build regular dCas9 inhibition-logic circuits. Due to time constraints, we couldn’t produce “double” activation/inhibition inputs. The response of the mutated promoter for single gRNA input, however, suggests that it behaves similarly to the original promoter.

Orthogonality : Transistors do not produce activation or inhibition patterns when the input gRNA is not complementary to their regulation sites

All our reporter + wrong gRNA constructs showed similar fluorescence levels. They are also very close to autofluorescence; they are in the “off” state. This result is consistent with studies of dCas9’s specificity in the literature.

Unfortunately, we lacked time to test specific regulation of one transistor among several within the same cell. Nevertheless, clues that multiple transistors can work simultaneously can be found in the chaining experiment below, where two non-orthogonal promoters worked in conjunction. The transistor response experiment is also informative on the orthogonality of signals as two gRNA inputs seem to have worked in parallel to produce the A/R1 response (see above). Moreover, the model predicted that despite sharing the apo-dCas9 the gRNA inputs are still able to form a sufficient amount of gRNA-dCas9 complexes to work as expected (LINK)

Chainability : One transistor can propagate signal to another transistor

Inducing the upstream transistor successfully resulted in induction of downstream transistor (A_u) to levels comparable to direct activation of J23117_alt (A_alt). We notice that the basal expression of the downstream reporter J23117_alt in chained configuration (B_d) is higher than the basal expression of regular J23117_alt (B_alt). This is probably due to leakage from the upstream promoter, as repressing the upstream promoter (R_u) resulted in lower fluorescent output. We therefore believe that our transistors could be chained to form more complex circuits. As far as we know, chaining dCas9-w-regulated promoters hadn’t been attempted. This might be the first exclusively dCas9-based bio-circuit in E. coli !

Supplementary findings

References

[1] Farzadfard, F., Perli, S. D., Lu, T. K. (2013). Tunable and Multifunctional Eukaryotic Transcription Factors Based on CRISPR/Cas. ACS Synth. Biol., 2 (10), pp 604–613.