Hybrid PKS-NRPS synthetase; part of the gene cluster that mediates the biosynthesis of equisetin, a trans-fused decalin-containing tetramic acid with antimicrobial activity (PubMed:23614392). The PKS module of eqxS together with the enoylreductase eqxC catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the eqxS NRPS module (PubMed:23614392). Activity of the Dieckmann cyclase domain (RED) results in release of the Dieckmann product intermediate (PubMed:23614392, PubMed:18652469). Diels-Alderase eqx3 is involved in endo-selective Diels-Alder cycloaddition to form the decalin ring, leading to the production of N-desmethylequisetin also called trichosetin (By similarity). Subsequent N-methylation is carried out by eqxD to give equisetin (PubMed:23614392). acetyl-CoA + ATP + 11 H(+) + L-serine + 7 malonyl-CoA + 8 NADPH + 2 S-adenosyl-L-methionine = (5S)-3-[(2E,6R,8E,10E,12E)-2,6-dimethyltetradeca-2,8,10,12-tetraenoyl]-5-(hydroxymethyl)pyrrolidine-2,4-dione + AMP + 7 CO2 + 8 CoA + diphosphate + 6 H2O + 8 NADP(+) + 2 S-adenosyl-L-homocysteine Mycotoxin biosynthesis. 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 (By similarity). 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 (By similarity). Occasionally, epimerase (E) domains (responsible for l- to d-amino acid conversion) are present within the NRP synthetase (By similarity). EqxS contains also a polyketide synthase module (PKS) consisting of several catalytic domains including a ketoacyl synthase domain (KS), an acyl transferase domain (AT), a dehydratase domain (DH), a methyltransferase domain (MT), and a ketoreductase domain (KR) (PubMed:23614392). Instead of a thioesterase domain (TE), eqxS finishes with a reductase-like domain (R) for peptide release (PubMed:23614392). EqxS has the following architecture: KS-AT-DH-KR-PCP-C-A-T-R (PubMed:23614392). In the C-terminal section; belongs to the NRP synthetase family.