All chemicals were purchased from Fisher Scientific or Aldrich unless noted otherwise. Peptide Calibration Standard II for MALDI-TOF MS was purchased from Bruker. mNisA, NisC, HalA2 and HalM2 were produced in Escherichia coli as previously described.13 (link),20a (link) Haloduracin β was purified from Bacillus halodurans C-125, as described previously.26 (link)