Catalyzes the oxidative decarboxylation of the C-terminal cysteine residue of mersacidin to an aminoenethiol residue. Binds 1 FAD per subunit. Antibiotic biosynthesis; mersacidin biosynthesis. Homododecamer. Belongs to the HFCD (homooligomeric flavin containing Cys decarboxylase) superfamily.