Part:BBa_K1895996

Red light sensitive 'LDR'

In addition to designing an arabinose controlled variable resistor for our iGEM work we also investigated alternative methods of controlling resistance that are found in common electronic components. One such example is the light dependent resistor, or LDR. Our biological LDR is based on the same scheme as our arabinose controlled resistor. That is, using E. coli to vary the amount of free ions in an electrolyte. Ion uptake will be controlled by the expression of a protein, smtA which is a metallothionein capable of binding to heavy metal ions like cadmium (II), Zinc (II) and Copper (II).

For this LDR we will be using the red light detection system from the Colliroid project. In this scheme the production of SmtA which affects the resistivity is placed under the control of the OmpF upstream promoter (BBa_R0084). Through the addition of a genetic invertor circuit we hope this part will form a system system where this promoter is repressed in the dark and has increased transcription under (red) light. This would the device to mimic the behaviour of a traditional electronic LDR whereby resistance is decreased in the light and increased in the dark.

We have also designed a blue light sensitive LDR using FixJ and YF1.

Usage and Biology

The OmpF promoter is repressed by phosphorylated OmpR, OmpR-P. In normal conditions the E. coli cell contains free OmpR which can be phosphorylated by expression of a protein with an EnvZ domain. One such protein is the fusion protein, Cph8 (BBa_I15010).

In order for the light responsive domain of the fusion protein cph8 to sense red light the formation of a chromophore is required this is done by the production of two proteins, ho1 and PcyA together with the cph8. In our system these will be constitutively expressed to create the red light sensor.

In the dark the fusion protein, Cph8 phosphorylates OmpR and thereby prevents SmtA production as the OmpF promoter is repressed. This increases resistance. In the light, the light responsive domain Cph1 inhibits the activity of the EnvZ is prevented from phosphorylating OmpR and therefore allows SmtA production and decreased resistance.

Through the addition of a basic genetic inverter circuit it would be possible to swap this behaviour so that the resistance decreases in the dark and increases in the light depending on the desired behaviour of your system.

Please note that this will only work in E. coli which are naturally deficient in EnvZ.