Hybrid PKS-NRPS synthetase; part of the gene cluster that mediates the biosynthesis of HIV-1 integrase inhibitor equisetin and of fusarisetin A, both trans-fused decalin-containing tetramic acids showing also antimicrobial activity (PubMed:25770422). The PKS module of fsa1 together with the enoylreductase fsa3 catalyze the formation of the polyketide unit which is then conjugated to L-serine by the condensation domain of the fsa1 NRPS module (PubMed:25770422). Activity of the Dieckmann cyclase domain (RED) results in release of the Dieckmann product intermediate (PubMed:25770422). Diels-Alderase fsa2 is involved in endo-selective Diels-Alder cycloaddition to form the decalin ring, leading to the production of N-desmethylequisetin also called trichosetin (PubMed:25770422, PubMed:28401214, PubMed:29972614, PubMed:34121297). Subsequent N-methylation is carried out by fsa4 to give equisetin (PubMed:25770422). The enzymatic gene responsible for the conversion of equisetin to fusarisetin A has not been identified yet and is probably located outside of the fsa cluster (PubMed:28401214). 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). Fsa1 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:25770422). Instead of a thioesterase domain (TE), fsa1 finishes with a reductase-like domain (R) for peptide release (PubMed:25770422). Fsa1 has the following architecture: KS-AT-DH-KR-PCP-C-A-T-R (PubMed:25770422). Results in the loss of production of equisetin and fusarisetin A (PubMed:25770422). In the C-terminal section; belongs to the NRP synthetase family.