Promoter, Repressor and Inducer Specification

• Promoter name: lacp (Lac repressible promoter)
• Length: 200 base pairs
• Strength with:
  • Weak RBS (BBa_B0032): 5.6 - 517.5 (+17) RPU
  • Medium RBS (BBa_B0030): 25.5 - 2323.1 (+259.5) RPU
  • Strong RBS (BBa_B0034): 39.6 - 4041.5 (+247.8) RPU

In gram negative bacteria, LacI represses lacp which regulates the binding of RNA polymerase and inhibits transcription by binding to the lac operon.

• Repressor name: LacI (LacI repressor)
• Part Length: 1153 base pairs
• Structure: Homo-tetramer
• Binding affinity: To lacp, very tight, () µl/ml has been shown to cause a 5 order of magnitude change in fluorescent protein production

The LacI repressor is an allosteric protein exists in 2 states: silent and active. In the absence of inducers, the active state of LacI binds to the lac operator sites of the lacp promoter and interferes the binding the RNA polymerase, thus inhibiting transcription. In the presence of inducers, however, LacI undergoes allosteric rearrangement and switches to its silent state, resulting in its release from the promoter, thus allowing transcription to take place.

• Inducer name: IPTG (Isopropyl β-D-1-thiogalactopyranoside)
• Binding Affinity: (to LacI) Ka ~ 109 M-1
• Effective range: 250 - 25000 ng/ml

In our project, Isopropyl β-D-1-thiogalactopyranoside (IPTG) was used to induce lacp driven expressions. IPTG is a lactose metabolite, lacp’s native inducer, which has shown stronger binding affinity to LacI. On top of that, it cannot be metabolized by Escherichia coli so it concentration remains constant during the experiment, making it a more suitable inducer for our tristable switch system.