Detection of intracellular arabinose with FLIPara-200n. (A) Detection of intracellular arabinose in Escherichia coli BL21-Gold(DE3) cells expressing FLIPara-250n. The arrow indicates arabinose addition (note that here the addition was performed by hand, leading to an interruption of the protocol, thus rates cannot be deduced; marked as a gray bar). Error bars indicate the standard deviation of at least three replicates. (B) Arabinose-dependent sensor in vivo response at t = 25 minutes. The intracellular arabinose concentration that corresponds to the affinity constant (Kd) of FLIParaF.Ec-250n is reached at an external arabinose concentration of 4.7 ± 2.0 μM, indicating that the intracellular concentration is around 20-fold lower compared with the extracellular concentration in a medium containing 4.7 mM arabinose. Error bars represent the standard deviation (n = 3).