L-Cysteine desulfurase IscS and scaffold IscU proteins are universally involved in Fe/S cluster synthesis. The Archaeoglobus fulgidus (Af) genome encodes proteins having a high degree of primary structure similarity to IscS and IscU from other organisms. However, AfIscS is unusual because it lacks the active site lysine residue that normally forms an internal Schiff base with pyridoxal-phosphate (PLP) and serves as a base during catalysis. Our as-isolated recombinant AfIscS contains pyridoxamine phosphate (PMP) instead of the expected PLP and lacks desulfurase activity. We have solved its structure to 1.43 Å resolution and found that PMP binds non-covalently at the PLP site of the enzyme and displays significant disorder. However, the previously reported structure of recombinant Af(IscU-D35A-IscS)(2) contains an in vivo generated [Fe(2)S(2)] species within AfIscU and the question arises as to how its sulfides were generated. Here, we report that adding PLP to AfIscS produces an enzyme that displays in vitro L-cysteine desulfurase activity mediating the synthesis of a stable holo Af(IscU-D35A-IscS) complex.