Cytochrome P450 monooxygenase; part of the gene cluster that mediates the biosynthesis of UCS1025A, a member of the pyrrolizidinone family that acts as a strong telomerase inhibitor and displays potent antibacterial and antitumor properties (PubMed:29373009). These compounds share a hemiaminal-containing pyrrolizidinone core fused with a gamma-lactone, giving a furopyrrolizidine that is connected to a decalin fragment (PubMed:29373009). The polyketide synthase module (PKS) of the PKS-NRPS ucsA is responsible for the synthesis of the polyketide backbone via the condensation of an acetyl-CoA starter unit with 6 malonyl-CoA units (PubMed:29373009). The downstream nonribosomal peptide synthetase (NRPS) module then amidates the carboxyl end of the polyketide with a 2S,3S-methylproline derived from L-isoleucine by the 2-oxoglutarate-dependent dioxygenase ucsF which converts L-isoleucine to (4S,5S)-4-methylpyrroline-5-carboxylate that is further converted to 2S,3S-methylproline by the pyrroline-5-carboxylate reductase ucsG (PubMed:29373009). Reductive release of the completed aminoacyl polyketide from the assembly line can form the 3-pyrrolin-2-one structure via an intramolecular Knoevenagel reaction (PubMed:29373009). Because ucsA lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase ucsL (PubMed:29373009). This keto acyclic precursor is the substrate of the Diels-Alderase ucsH, that catalyzes the Diels-Alder cycloaddition (PubMed:29373009). Oxidation of the 3S-methyl group to a carboxylate by the cytochrome P450 monooxygenase ucsK allows an oxa-Michael cyclization that might involve the reductase/dehydrogenase ucsI and which furnishes the furopyrrolizidine (PubMed:29373009). The oxidase ucsJ likely plays a critical role in stereoselective reduction of the C5-C6 double bond to afford the required R-configured carboxylate group (Probable). Further enolization and oxidation at C5 by an unidentified enzyme affords the last intermediate that can undergo oxa-Michael cyclization to yield UCS1025A (Probable). Mycotoxin biosynthesis. Leads to the accumulation of intermediate diastereomers that are methylated at C6 of the pyrrolizidinone. Belongs to the cytochrome P450 family.