Involved in 'Ser-type' sulfatase maturation under anaerobic conditions. Catalyzes the post-translational modification of serine ('Ser-72' in the arylsulfatase AtsA) 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:10336424, PubMed:12419807, PubMed:14749327, PubMed:18558715). Is capable of catalyzing multiple turnovers (PubMed:18558715). In vitro, use of a peptide substrate in which the target serine is changed to cysteine also gives rise to turnover, supporting approximately 4-fold the activity of that observed with the natural substrate (PubMed:18558715). L-seryl-[sulfatase] + S-adenosyl-L-methionine = 3-oxo-L-alanyl-[sulfatase] + 5'-deoxyadenosine + H(+) + L-methionine Binds 3 [4Fe-4S] clusters (PubMed:18558715). The first cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine (By similarity). Activity is inhibited by iron chelators. Protein modification; sulfatase oxidation. Monomer (PubMed:23477283). Interacts with AtsA prior to its export to the periplasm (PubMed:12419807). This interaction depends on the presence of AtsA 'Ser-72' (PubMed:12419807). Binding of SAM to AtsB promotes the formation of a ternary AtsA-AtsB-SAM complex (PubMed:14749327). Belongs to the radical SAM superfamily. Anaerobic sulfatase-maturating enzyme family.