Rv1739c STAS binds guanine nucleotides. A, photoaffinity labeling of Rv1739c STAS with 8-N3-GTP γ-biotinyl-LC-PEO-amine, in the absence and presence of 2 mm GTP. A representative of three similar experiments is shown. B, quench of intrinsic fluorescence of Rv1739c STAS (12–20 μm) by the indicated concentrations of GDP. C, concentration dependence for quench of the Rv1739c STAS peak intrinsic fluorescence by GDP (K½ = 146 ± 38 μm; r2 = 0.98). D, concentration dependence for quench of Rv1739c STAS peak fluorescence by GTP (K½ = 347 ± 227 μm; r2 = 0.96). λex was 280 nm; λem was scanned between 300 and 400 nm. Values in C and D are mean ± S.E. (n ≥ 3).