Thiostrepton (TSR), an archetypal member of the family of ribosomally synthesized and post-translationally modified thiopeptide antibiotics, possesses a biologically important quinaldic acid (QA) moiety within the side-ring system of its characteristic thiopeptide framework. QA is derived from an independent L-Trp residue; however, its associated transformation process remains poorly understood. We here report that during the formation of QA, the key expansion of an indole to a quinoline relies on the activities of the pyridoxal-5'-phosphate-dependent protein TsrA and the flavoprotein TsrE. These proteins act in tandem to process the precursor 2-methyl-L-Trp through reversible transamination and selective oxygenation, thereby initiating a highly reactive rearrangement in which selective C2-N1 bond cleavage via hydrolysis for indole ring-opening is closely coupled with C2'-N1 bond formation via condensation for recyclization and ring expansion in the production of a quinoline ketone intermediate. This indole ring-expansion mechanism is unusual, and represents a new strategy found in nature for L-Trp-based functionalization.