ESCHERICHIA-COLI CONTAINS A PROTEIN THAT IS HOMOLOGOUS IN FUNCTION AND N-TERMINAL SEQUENCE TO THE PROTEIN ENCODED BY THE NIFS GENE OF AZOTOBACTER-VINELANDII AND THAT CAN PARTICIPATE IN THE SYNTHESIS OF THE FE-S CLUSTER OF DIHYDROXY-ACID DEHYDRATASE

In this paper, I report the purification of a protein from Escherichia coil that is very similar in sequence, molecular weight, and the reac tions it can catalyze to the protein encoded by the Azotobacter vinela ndii nifS gene. This E. coli protein contains pyridoxal phosphate as a cofactor and catalyzes the removal of sulfur from cysteine to form al anine and S-0. When dithiothreitol is present along with cysteine, the S-0 formed is reduced to S2-. This protein has a reactive sulfhydryl group that is essential for activity. As isolated, this sulfhydryl gro up appears to be in a disulfide linkage with the sulfhydryl group from the phosphopantetheine moiety of the acyl carrier protein. The purifi ed E. coli protein can mobilize the sulfur from cysteine and contribut e it to the formation of a [4Fe-4S] cluster on the apoprotein off. col i dihydroxy-acid dehydratase. A mechanism is proposed for the early st ages of the synthesis of Fe-S clusters using this protein and sulfur i n the S-0 oxidation state.