Nonribosomal peptide synthetase; part of the gene clusters that mediate the biosynthesis of AM-toxins, host-selective toxins (HSTs) causing Alternaria blotch on apple, a worldwide distributed disease (Probable). AM-toxins are cyclic depsipeptides containing the 3 residues 2-hydroxy-isovaleric acid (2-HIV), dehydroalanine, L-alanine which are common for all 3 AM-toxins I to III. The fourth precursor is L-alpha-amino-methoxyphenyl-valeric acid (L-Amv) for AM-toxin I, L-alpha-amino-phenyl-valeric acid (L-Apv) for AM-toxin II, and L-alpha-amino-hydroxyphenyl-valeric acid (L-Ahv) for AM-toxin III (Probable). AM-toxins have two target sites for affecting susceptible apple cells; they cause invagination of the plasma membrane and electrolyte loss and chloroplast disorganization (PubMed:22846083). The non-ribosomal peptide synthetase AMT1 contains 4 catalytic modules and is responsible for activation of each residue in AM-toxin (PubMed:10875335). The aldo-keto reductase AMT2 catalyzes the conversion of 2-keto-isovaleric acid (2-KIV) to 2-hydroxy-isovaleric acid (2-HIV), one of the precursor residues incorporated by AMT1 during AM-toxin biosynthesis, by reduction of its ketone to an alcohol (PubMed:15066029). The cytochrome P450 monooxygenase AMT3 and the thioesterase AMT4 are also important for AM-toxin production, but their exact function within the AM-toxin biosynthesis are not known yet (PubMed:17990954). Up to 21 proteins (including AMT1 to AMT4) are predicted to be involved in AM-toxin biosynthesis since their expression ishighly up-regulated in AM-toxin-producing cultures (PubMed:17990954). Mycotoxin biosynthesis. Expression is up-regulated more than 10 fold in toxin producing cultures. NRP synthetases are composed of discrete domains (adenylation (A), thiolation (T) or peptidyl carrier protein (PCP) and condensation (C) domains) which when grouped together are referred to as a single module. Each module is responsible for the recognition (via the A domain) and incorporation of a single amino acid into the growing peptide product. Thus, an NRP synthetase is generally composed of one or more modules and can terminate in a thioesterase domain (TE) that releases the newly synthesized peptide from the enzyme. Occasionally, epimerase (E) domains (responsible for L- to D-amino acid conversion) are present within the NRP synthetase (By similarity). AMT10 is composed of only one module and misses a condensation (C) domain (Probable). Gene clusters encoding host-selective toxins (HSTs) are localized on conditionally dispensable chromosomes (CDCs), also called supernumerary chromosomes, where they are present in multiple copies (PubMed:17990954). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:17990954). Belongs to the NRP synthetase family.