Iron-sulfur proteins acquire their clusters by posttranslational assembly. To identify components involved in this process an in vitro assay for holoprotein formation was established using the [2Fe-2S] ferredoxin of the cyanobacterium Synechocystis as a model. Conversion of apoferredoxin to the holo- form was observed in an anaerobic reaction medium containing Fe(NH4)2(SO4)2, L-cysteine, glutathione, and catalytic amounts of Synechocystis extract, specifically depleted of endogeneous ferredoxin. An approximate 2500-fold purification of the converter activity yielded a monomeric, 43-kDa, pyridoxal phosphate-containing enzyme, which catalyzed the breakdown of L-cysteine to yield sulfide (assembled in ferredoxin), pyruvate, and ammonia; 1 mol of [2Fe-2S] ferredoxin was formed per 2 mol of cysteine utilized. The purified enzyme also catalyzed the beta-elimination reaction with cysteine in the absence of apoferredoxin. An increased reactivity was found with cystine instead of cysteine, which should yield cysteine persulfide as the primary product. These results provide a function-based identification of a cysteine/cystine C-S-lyase as a participant in ferredoxin Fe-S cluster formation. A substrate-derived cysteine persulfide could be involved in this reaction.