Team:ETH Zurich/Switch Module

PAVLOV'S COLI


SWITCH MODULE

OVERVIEW

Figure 1: The memory element of our circuit: the switch module.

The switch is the memory element of our circuit. We designed and tested different variants, all of them are based on editing of the plasmids or of the chromosome.

QUICK LINKS

These links will quickly take you to the aspects of our work you may be interested in:

GOALS

  • Provide qualitative information for the design of the switch.
  • Maximize the ratio between true and false positives.
  • Show if the population effect can be used for retrieving quantitative informations.

MODEL

We designed and modeled several variants of the switch.

VERSION 1: BXB1, TP901 AND PHIC31 INTEGRASES

Figure 1: Integrase switch design, the same design applies to Bxb1, Tp901 and PhiC31. Click to enlarge.

The simplest version of the switch is based on the integrase family of recombinases. On this page we focus on the Bxb1 integrase, but the same model and analysis applies to Tp901 and PhiC31.

From the literature1, the dynamics of Bxb1 appear to be well known. When the AND gate in the sensor module activates, Bxb1 is expressed and dimerizes. In its dimerized form, Bxb1 can bind to the attB and attR binding sites placed around the Pout promoter. When both sites are occupied, synapsis between the two can happen, enabling flipping.

The flipping process changes the direction of the reporter promoter and transforms the attB, attP sites in attL, attR. At this point the switch is in the ON state, Bxb1 dimers can still bind to the attL and attR sites, but flipping the sequence again.

The following section describes the species and reactions involved.

REACTIONS

SPECIES

Name Description
PhybON Fraction of activity of the AND gate hybrid promoter
mRNAint mRNA of the integrase
Bxb1 Integrase protein
DBxb1 Dimerized form of the integrase protein
S0 Plasmid with free attB and attP sites.
S1 Plasmid with DBxb1 bound to the attB or attP site.
S2 Plasmid with DBxb1 bound to both the attB and attP sites.
Pswitched Plasmid with switched reporter promoter.

VERSION 2: CRISPR/CAS9 CHROMOSOMAL GENE EDITING

Figure 1: CRISPR/Cas9 switch design. Click to enlarge.

The first one of our CRISPR switches is based on the Cas9 system.

VERSION 3: CRISPR/CPF1 CHROMOSOMAL GENE EDITING

Figure 1: CRISPR/Cpf1 switch design. Click to enlarge.

The last variant is based on CRISPR/Cpf1.

RESULTS

POPULATION EFFECT ANALISYS

TUNING FOR THE TARGET TIMESCALE

In the context of IBD investigation, we know that the signals we want to detect in the gut last for 2 to 6 hours. Thus it is desirable that the system requires the same amount of time to reach full switching.

The parameters that can be easily modified in the biological implementation are the invertase translation rate (RBS engineering) and degradation rate (degradation tags). A senstivity analysis (PLOT) showed that these parameters are indeed sensitive to variations, and their adjustment can make the system work optimally.

PLACEMENT OF THE INTEGRASE GENE

For the integrase version of the switch we evaluated two different designs trying to reduce the effects of the leakiness caused by the AND gate:

  1. The integrase gene is placed outside the flipping cassette.
  2. The integrase gene is placed inside the flipping cassette.

In the latter case the expression of the invertase protein decreases when plasmids start to switch. An early qualitative simulation showed that option 2 slightly reduces leakiness, but it also make complete flipping almost impossible. We conclude that option 1 is preferrable. (PLOT)

CHOICE OF PLASMID ORIs

REFERENCES

  • Singh, Shweta, Pallavi Ghosh, and Graham F. Hatfull. "Attachment site selection and identity in Bxb1 serine integrase-mediated site-specific recombination." PLoS Genet 9.5 (2013): e1003490.

Thanks to the sponsors that supported our project: