The rational regulation of protein concentration remains an elusive problem in synthetic biology. Several strategies have been developed at the transcriptional level, but these suffer from suboptimal response kinetics due to slow turnover of any protein already synthesized. The direct fusion of degradation tags to a protein of interest (POI) results in rapid fluctuations in protein concentration, and several degradation tags have been developed that function conditionally on the presence of a small molecule. However, no solutions currently exist that allow the intracellular concentration of a POI to be regulated by the cellular environment.
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