Oxidase; part of the gene cluster that mediates the biosynthesis of the secondary metabolite ustiloxin B, an antimitotic tetrapeptide (PubMed:24841822, PubMed:27166860, PubMed:26703898). First, ustA is processed by the subtilisin-like endoprotease Kex2 that is outside the ustiloxin B gene cluster, at the C-terminal side of Arg-Lys, after transfer to Golgi apparatus through the endoplasmic reticulum (ER) (PubMed:24841822). Cleavage by KEX2 generates 16 peptides YAIG-I to YAIG-XVI (PubMed:24841822). To process the precursor peptide further, at least two peptidases are necessary to cleave the N-terminal and C-terminal sides of the Tyr-Ala-Ile-Gly core peptide which serves as backbone for the synthesis of ustiloxin B, through cyclization and modification of the tyrosine with a non-protein coding amino acid, norvaline (PubMed:24841822). One of the two peptidases must be the serine peptidase ustP; and the other pepdidase is probably ustH (PubMed:24841822). Macrocyclization of the core peptide derived from ustA requires the tyrosinase ustQ, as well as the homologous oxidases ustYa and ustYb, and leads to the production of the first cyclization product N-desmethylustiloxin F (PubMed:27166860, PubMed:26703898). For the formation of N-desmethylustiloxin F, three oxidation steps are required, hydroxylation at the benzylic position, hydroxylation at either the aromatic ring of Tyr or beta-position of Ile, and oxidative cyclization (PubMed:27166860). UstQ may catalyze the oxidation of a phenol moiety, whereas the ustYa and ustYb are most likely responsible for the remaining two-step oxidations (PubMed:27166860). N-desmethylustiloxin F is then methylated by ustM to yield ustiloxin F which in turn substrate of the cytochrome P450 monooxygenase ustC which catalyzes the formation of S-deoxyustiloxin H (PubMed:27166860). The flavoprotein monooxygenases ustF1 and ustF2 then participate in the modification of the side chain of S-deoxyustiloxin H, leading to the synthesis of an oxime intermediate, via ustiloxin H (PubMed:27166860). Finally, carboxylative dehydration performed by the cysteine desulfurase-like protein ustD yields ustiloxin B (PubMed:27166860). Mycotoxin biosynthesis. Decreases the production of ustiloxin B (PubMed:24841822, PubMed:26703898).