To produce the antibiotic Microcin B17, four Cys and four Ser residues are converted into four thiazoles and four oxazoles by the three subunit Microcin B17 synthetase. High-resolution mass spectrometry (MS) was used to monitor the kinetics of posttranslational heterocyclic ring formation (-20 Da per ring) and demonstrated the accumulation of all intermediates, from one to seven rings, indicating distributive processing. All of the intermediates could be converted by the enzyme to the eight ring product. Enzymatic chemoselectivity (Cys vs Ser cyclization rates) was assessed using iodoacetamido-salicylate to alkylate unreacted cysteines (+193 Da) in the 8 kDa biosynthetic intermediates; three of the first four rings formed were thiazoles, and by the five ring stage, all four of the cysteines had been heterocyclized while three of the original four serines remained uncyclized. Finally, tandem MS using a 9.4 T Fourier transform instrument with electrospray ionization was used to elaborate the major processing pathway: the first two rings formed are at the most amino proximal sites (Cys(41) then Ser(40)) followed by the remaining three cysteines at positions 48, 51, and 55. The cyclization of serines at positions 56, 62, and 65 then follows, with Ser(62) and Ser(65) the last to heterocyclize and the first of these at a slower rate. Thus, despite free dissociation of intermediates after each of seven ring-forming catalytic cycles, there is an overall directionality of ring formation from N-terminal to C-terminal sites. This remarkable regioselectivity is determined more by the substrate than the enzyme, due to a combination of (1) initial high-affinity binding of the posttranslational catalyst to the N-terminal propeptide of substrate 88mer, and (2) a chemoselectivity for thiazole over oxazole formation. This mechanism is consistent with antibiotic biosynthesis in vivo, yielding microcin with six, seven, and eight rings, all with bioactivity.