This year, we want to offer a handy, adjustable and useful tool kit to everyone working on Synthetic Biology--Signal Filter. The core circuit is based on a positive-feedback bi-stable or tri-stable system with the capacity of reducing noise and converting pulse signal into robust and persistent signal. To adapt to different experimental requirements, we developed two versions of our Signal Filter: prokaryotic one and eukaryotic one.
It is a tri-stable system adapted from bacteriophage λ operon. In the operon, promoter RE is activated by CII trascriptional activator. Ftsh is an ATP-dependent host metalloprotease which will normally degrade CII, while CIII serves as an inhibitor of Ftsh to free CII. CI can function as an inhibitor to block pR, while Cro can bind pRM to stop the expression of downstream gene. The positive feedback control under pRE is used to enhance pulse signal 1 and convert it into robust stable signal.
Fig 1: circuit of prokaryote version serving as tri-stable signal filter
When pulse 1 is sensed, a double promoter structure will help induce cro and cII downstream pRE. At the same time constitutively produced CIII will guarantee enough CII to enhance the downstream transcription of the pRE. Thus, there forms a positive feedback loop to direct a fast and strong expression of cro. A quantity of Cro represses transcription under pRM by binding to cro binding site and blocks gene of interest 2’s expression, so it turns to a stable state of expressing gene of interest 1.
When pulse 2 comes, under a certain inducible promoter, CI will be expressed and then binds to CI binding site within pR promoter, blocking gene of interest 1 and CIII’s expressions. With CIII’s reduction, FtsH gradually degrades CII to interrupt the stable state. Therefore, expression of cro immediately drops down, allowing transcription from pRM. The system turns to another stable state of gene of interest 2’s expression.
If there is no signal. Both genes of interest will be expressed. And if both of the two signal exist, the expression state will depend on the intensity of the initial signal input.
It is a bi-stable system derived from Arabidopsis thaliana stress response system. Enzyme catalysis is the core of this design to increase the efficiency of state transition.
Fig 2: circuit of Eukaryote version serving as bii-stable signal filter
Clade A protein phosphatases type 2C (PP2CA) and Sucrose Nonfermenting1 Related Subfamily 2 (SnRK2s) protein kinase are both components of Abscisic acid signaling network in Arabidopsis thaliana. ABF2 is a leucine zipper transcription factor which basically binds ABA-response element (ABRE). ABF2 can be phosphorylated by SnRK2s and be efficiently dephosphorylated by PP2CA. Also, in the absence of Abscisic acid, SnRK2 kinases can be inactivated by PP2Cs thus shut down the gene expression efficiently.
When signal 1 (ON) comes, promoter RD29A drives expression of SnRK2.2, which can phosphorylate ABF2 (constitutively expressed). Compared with protein co-facter association, enzyme catalysis can produce a large quantity of phosphorylated ABF2 in a short time, then enhance pRD29A to turn on the gene of interest’s expression. When signal 2 (OFF) comes, gene PP2CA expresses, dephosphorylates ABF2 and inactivates SnRK2.2, thus turning the system into OFF state.