Part:BBa_K1895997

Blue 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 a blue light sensitive protein system. In this scheme the production of SmtA which affects the resistivity is placed under the control of of a FixJ-P (phosphorylated FixJ) promoter. This allows the protein production to be regulated by blue light through a series of reactions with the FixJ response regulator protein YF1.

We have also designed a red light sensitive LDR using OmpR.

Usage and Biology

In the absence of light, YF1 undergoes autophosphorylation to produce YF1-P which can phosphorylate FixJ. This in turn activates the transcription of the downstream protein. In this case that is SmtA. Thus, in the presence of light SmtA is not produced and so conductivity does not change, whilst in the absence of light SmtA is produced resulting in a decrease in resistance.

This behaviour is the inverse of an electrical light dependent resistor where resistance increases with light intensity. To mimic this behaviour using biological circuits we would place an inverter before the FixK2 promoter (which is activated by FixJ-P). The inverter is constructed by placing the desired output, here SmtA, under the control of a lambda cl regulated promoter (BBa_R0051). As lambda cl represses the promoter, having the lambda cl protein itself produced under control of FixK2 promoter inverts the system. This results in SmtA being produced in the presence of light rather than the absence thereof. BBa_K592020 is an example of a part that uses this technique.