IscS plays a principal role in the synthesis of sulfur-containing biomolecules. It is known that the expression of iscS can be negatively regulated by IscR, the first gene product of iscRSUA-hscBA-fdx. What governs the regulation of cysteine desulfurase activity, however, is unknown. Here, we report that IscS from Escherichia coli is able to bind iron with an association constant of 1.6x10(17)M(-1) to form an IscS-iron complex. IscS is also capable of binding both iron and sulfide to form an IscS-iron-sulfide complex with a higher affinity. The desulfurase activity is gradually inhibited as the amount of iron and sulfide bound to IscS increases. When 2Fe-2S binds IscS, about 20% of the activity is inhibited; when 8Fe-8S adheres to IscS, about 70% of the activity is inhibited. Thus, the cell is able to modulate its desulfurase activity with the formation of an IscS-iron-sulfide complex.