4-Nitrotryptophan is a substrate for the non-ribosomal peptide synthetase TxtB in the thaxtomin A biosynthetic pathway

Thaxtomin A, a cyclic dipeptide with a nitrated tryptophan moiety, is a phytotoxic pathogenicity determinant in scab-causing Streptomyces species that inhibits cellulose synthesis by an unknown mechanism. Thaxtomin A is produced by the action of two non-ribosomal peptide synthetase modules (TxtA and TxtB) and a complement of modifying enzymes, although the order of biosynthesis has not yet been determined. Analysis of a thaxtomin dual module knockout mutant and single module knockout mutants revealed that 4-nitrotryptophan is an intermediate in thaxtomin A biosynthesis prior to backbone assembly. The 4-nitrotryptophan represents a novel substrate for non-ribosomal peptide synthetases. Through identification of N-methyl-4-nitrotryptophan in a single module knockout and the use of adenylation domain specificity prediction software, TxtB was identified as the non-ribosomal peptide synthetase module specific for 4-nitrotryptophan.