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Team:Brown/Project/Light pattern/Logic design
From 2010.igem.org
Light-Pattern Controlled Circuit
Logic Design
Contents |
Before we begin our discussion of the circuit logic, let us first review what must be accomplished by this circuit:
- Input
- As explained in the Overview section, we desire the input to the cell to be light. Because light cannot alone activate or repress a promoter, the circuit must in some way be able to translate the light signal to a molecule that can regulate transcription. Additionally, using light as input implies a change in light state - from OFF to ON or vise versa. Thus, the molecule that provides the interface between light and the circuit must be able to switch between active and inactive states.
- Processing
- Given a molecule that toggles between active and inactive, it is easy to imagine a circuit that switches between one state of production and a second. However, we desire to achieve a progression of four states! In order to accomplish this, our circuit, unlike the light responsive molecule, cannot return to its original state when the light turns back OFF. Clearly, our circuit must be able to hold "memory!"
- Output
- We want our circuit to have distinct states - when possible, we would like a state to turn off after a new state has been turned on (with the exception of the memory, of course!).
Consideration of these requirements clearly gives rise to a handful of modules. The first is the Conversion Module, which takes light ON/OFF and converts it to transcription ON/OFF. This module involves a molecule that is directly modified by the presence or absence of light, and a component that effects a transcriptional change depending on the state of the light responsive molecule.
Second is the Memory Module, which "remembers" that light has been changed in the past. This Memory Module is a crucial way-point in the circuit, as without it the circuit would merely return to its original state when the light turns back OFF.
Third is the IF&NOT Module that generates the third state. As this module can only be active following the first and second states, its activation must depend on the presence of memory ON and the absence of some factor produced only when the light is some α state.
Finally, the last piece of the circuit is the AND Module, which produces the fourth and final state in our circuit. This module uses AND logic because it requires two inputs: one that is only expressed when the third state is active and a second that is expressed when the light is in some β state (the opposite of the previous light state, for the user has inputted a change!).
