Involved in 'Cys-type' sulfatase maturation under anaerobic conditions. Catalyzes the post-translational modification of cysteine into 3-oxoalanine (also known as C(alpha)-formylglycine (FGly)), by a free radical chemical mechanism initiated via the reductive cleavage of S-adenosyl-L-methionine (SAM). H2O + L-cysteinyl-[sulfatase] + S-adenosyl-L-methionine = 3-oxo-L-alanyl-[sulfatase] + 5'-deoxyadenosine + 2 H(+) + hydrogen sulfide + L-methionine Binds 3 [4Fe-4S] clusters (By similarity). The first cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine (By similarity). Protein modification; sulfatase oxidation. Belongs to the radical SAM superfamily. Anaerobic sulfatase-maturating enzyme family.