In this example of a typical transducer, the genetic construct (top) includes an amplifier response element and a carefully positioned transcription factor response element upstream of a promoter that drives expression of a fusion gene (the combined effector of choice and an amplifier). In the presence of a specific endogenous transcription factor, which binds to the transcription factor response element, the fusion gene is expressed. Cleavage of the fusion protein releases the amplifier, which together with the transcription factor drives much stronger expression. The system is like a positive feedback loop, but neither the transcription factor nor the amplifier alone can drive strong expression—they need each other.
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