Identification and stereochemical assignment of the beta-hydroxytryptophan intermediate in the echinomycin biosynthetic pathway.

[reaction: see text] Little is known about how quinoxaline-2-carboxylic acid (QC) is synthesized in nature. On the basis of analysis of echinomycin biosynthetic gene clusters as well as feeding experiments with labeled precursors, we have proposed a biosynthetic pathway to QC and identified the (2S,3S)-beta-hydroxytryptophan as a key intermediate.