Submitted to: Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2009
Publication Date: 10/5/2009
Citation: Johnson, E.G., Krasnoff, S., Bignell, D.R., Chung, W., Tao, T., Parry, R.J., Loria, R., Gibson, D.M. 2009. 4-Nitrotryptophan is a substrate for the non-ribosomal peptide synthetase TxtB in the thaxtomin A biosynthetic pathway. Molecular Microbiology. 73:409-418. Interpretive Summary: Several Streptomyces species cause scab diseases on potato due to the presence of a family of phytotoxins, the thaxtomins, which inhibit cellulose biosynthesis by some unknown mechanism and thus cause damaging corky lesions on tubers. Chemically thaxtomins can be thought of as a combination of two common amino acids (the building blocks of proteins), tryptophan and phenylalanine, both of which are slightly modified in the final thaxtomin molecule. The tryptophan is unusual because it has a nitrate group attached to it which is added by the action of a gene identified and described in a previous study. This work continues our investigation of the genes that control the biosynthesis of the thaxtomins. By analysis of the chemical products of gene knockout strains of thaxtomin-producing Streptomyces, this work shows that nitrotryptophan, itself, is a key intermediate in thaxtomin biosynthesis and is the substrate of the enzyme that adds the phenylalanine group to produce the full chemical skeleton of the thaxtomins This work continues our investigation of the genes that control the biosynthesis of the thaxtomins.
Technical Abstract: 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.