Involved in 'Cys-type' sulfatase maturation under anaerobic conditions. Catalyzes the post-translational modification of cysteine ('Cys-51' in the arylsulfatase CPF_0221) 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) (PubMed:18408004, PubMed:16766528, PubMed:17335281, PubMed:19489556, PubMed:23477283). Is also able to oxidize a serine residue in a synthetic substrate to FGly in vitro, and in a serine variant of a Cys-type sulfatase in vivo, but this activity is not physiological (PubMed:18408004, PubMed:23477283). Converts threonyl peptides to the corresponding ketone product, and also allo-threonyl peptides, but with a significantly reduced efficiency (PubMed:23477283). 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 (PubMed:23477283, PubMed:23650368). The first cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine (PubMed:23650368). The two auxiliary clusters may create a conduit for electrons to travel from the buried substrate to the protein surface (PubMed:23650368). Protein modification; sulfatase oxidation. Monomer. Recognizes the CXAXR sulfatase signature, in contrast to the oxygen-dependent formylglycine-generating enzyme system (FGE) that recognizes the CXPXR motif. Belongs to the radical SAM superfamily. Anaerobic sulfatase-maturating enzyme family.