To determine the pH rate profile of OphAΔC6, OphAΔC6-2h (4 mg/ml) was incubated with 5 mM SAM in appropriate buffers with a pH range from 4 to 10 (pH 4.0 to 5.0, 50 mM sodium citrate; pH 6.0 to 7.5, 50 mM bis-tris; pH 7.5 to 9.0, 50 mM tris; pH 9.0 to 10, 50 mM CHES). OphA precipitated at pH 4.0 and 5.0 sodium citrate buffer immediately, and reaction solution turned turbid after a 15-hour incubation at pH 6.0 bis-tris and pH 7.0 tris-HCl. The reactions were quenched after 15 hours by adding an equivalent volume of 8 M urea and further incubated for 1 hour. To make sure 4 M urea quenches the reaction, native OphAΔC6-2h was treated with 4 M urea and 5 mM SAM in tris-HCl for 1 hour. The samples were then diluted four times into ammonium bicarbonate (pH 8.5) and digested overnight with trypsin at 37°C (fig. S2B).

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