(A) SDS-PAGE mNisA ring variants incorporating 1. Lane 1: ring A, lane 2: ring B, lane 3: ring C. (B-D) QTOF-ESI for mNisA ring variants incorporating 1: (B) Ser3TAG, ring A. (C) Thr8TAG, ring B. (D) Thr13TAG, ring C. (E)13C-NMR of labeled 1 (red) and 2 (green). (F) 13C-NMR mNisA Cys to Ala mutants incorporating labeled 1, non-cyclized (red) and cyclized (green) peptides. Asterisks indicate GSH addition products.