With in vitro biochemical studies showing that βes can be mistranslated for Thr and selectively edited by BesG, we sought to evaluate the ability of BesG to prevent in vivo misincorporation of βes in the native producer S. cattleya. We therefore disrupted the besG gene in S. cattleya and supplied βes externally to WT S. cattleya and S. cattleya ΔbesG strains through the addition of spent media during exponential growth (Extended Data Figure 2). The proteomes were extracted and derivatized using the copper-catalyzed azide-alkyne cycloaddition (CuAAC) to a TAMRA fluorophore to label detection of βes incorporation by fluorescence. The fluorescence of the ΔbesG knockout strain was notably higher than WT, demonstrating that the absence of BesG appears to increase mistranslation at physiological concentrations of βes (Fig. 2D). Additional proteomics studies confirm that βes is found at Thr sites in the S. cattleya ΔbesG knockout strain (Table S2, Fig. S5).