Nonribosomal peptide synthetase; part of the gene cluster that mediates the biosynthesis of acetylaranotin, a member of the epipolythiodioxopiperazine (ETP) class of toxins characterized by a disulfide-bridged cyclic dipeptide (PubMed:23586797). The first step of acetylaranotin biosynthesis is performed by the NRPS ataP which produces diketopiperazine cyclo-L-Phe-L-Phe via the condensation of 2 phenylalanines (L-Phe) (PubMed:23586797). The ataC domain of ataTC then catalyzes the formation of bishydroxylation of cyclo-L-Phe-L-Phe (PubMed:23586797). The glutathione S-transferase domain ataG in ataIMG further catalyzes the conjugation of two glutathiones to the bishydroxylated intermediate (PubMed:23586797). Next, the dipeptidase ataJ removes the Glu residues (PubMed:23586797). The following step is performed by the carbon sulfur lyase domain ataI of ataIMG which may convert the bis-cysteinyl adduct to yield an epidithiol intermediate (PubMed:23586797). The ataT domain from ataTC then catalyzes the oxidation of the free dithiols, followed by a cyclization step catalyzed by the cytochrome P450 ataF (PubMed:23586797). AtaF probably acts as an epoxidase to promote a dual epoxidation formation at C8 and C9 along with C8' and C9', followed by the spontaneous nucleophilic attack of the amide nitrogens N10 and N10' to yield an intermediate with the pyrrolidine partial structure (PubMed:23586797). The final steps of acetylaranotin biosynthesis involve the acetylation and ring rearrangement of an epitetrathiodiketopiperazine intermediate to produce acetylaranotin (PubMed:23586797). AtaH probably catalyzes the acetylation of epitetrathiodiketopiperazine to produce a diacetate and ataY is responsible for the formation of the dihydrooxepin moiety that converts the diacetate intermediate to acetylaranotin via acetylapoaranotin (PubMed:23586797). Both enzymes could function independently in the absence of the other (PubMed:23586797). The specific function of ataL within the pathway has still to be determined (PubMed:23586797). The acetylaranotin bis-thiomethyltransferase ataS located outside of acetylaranotin gene cluster is the main thiomethyltransferase responsible for converting acetylaranotin and its related intermediates to their methylated forms (PubMed:30096370). Mycotoxin biosynthesis. Impairs the production of acetylaranotin and accumulates chemically stable intermediate cyclo-L-phe-L-phe or shunt products such as 2-hydroxy-2'-ene-cyclo-L-Phe-L-Phe and 2-imino-10'-hydroxy-cyclo-L-Phe-L-Phe (PubMed:23586797).