Q-TOF MS detection of the 26-residue tryptic PKS fragment that harbors the serine active site of the ACP domain. Top panel, the tryptic fragment from the PKS produced in E. coli, with m/z 928.1374 for [M+3H]3+ (calculated 928.1604); mid panel, the tryptic fragment from the 4′-phosphopantetheinylated PKS, with m/z 1041.5487 for [M+3H]3+ (calculated 1041.5221); bottom panel, the tryptic fragment from acylated PKS, with m/z 1113.4847 for [M+3H]3+ (calculated 1113.5533), showing the hexaketide polyene synthesized by the single-module PKS.