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Reporter assay to assess GppX as a transcription regulator. The β-galactosidase assay was performed on E. coli strains MG1655 (positive control), pAC-lacTFggpΔHTH (negative control), pACYC184 (empty vector), and pAC-lac-gppX (in trans). Activity was determined by chromogenic O-nitrophenyl-β-D-galactoside (ONPG) substrate with, or without induction of isopropyl β-D-1-thiogalactopyranoside (IPTG) and measured using Miller Units. Each value represents the mean (±SD) of three values measured in one representative assay. Data are representative of three independent experiments with similar results, ∗∗∗P < 0.001.