Ferredoxin-thioredoxinreductase is an essential member of a light-dependent regulatory system in oxygenic photosynthesis. This enzyme is composed of two nonidentical subunits, contains a 4Fe-4S cluster of a yet unknown function, and a catalytically active dithiol group. We have used
 N-ethylmaleimide to modify specifically the active site thiols of the protein and show that their chemical modification completely abolishes the catalytic capacity of the enzyme and causes changes in the absorption spectrum of the protein