To determine whether the aerobic and anaerobic respiratory pathways of Escherichia coli are regulated in response to iron availability, strains containing lacZ reporter fusions to the cydAB, cyoABCDE, narGHJI, dmsABC and frdABCD operons were grown in medium limited for iron by use of the chelator, 2,2'-dipyridyl. Under anaerobic conditions, expression of the anaerobic respiratory pathway operons, narG-lacZ, dmsA-lacZ and frdA-lacZ, was reduced 14-16 fold when iron was limited. In contrast, expression of the aerobic pathway operons, cyoA-lacZ and cydA-lacZ, was elevated modestly. Iron-dependent transcriptional control of these operons was independent of the fur gene which encodes an iron-and-DNA-binding regulatory protein. The expression of fnr-lacZ was relatively unaffected by iron limitation suggesting that Fnr levels in the cell do not change in response to iron. The above findings suggest that in addition to Fur, some other cellular protein may bind iron for reporting and regulating iron-dependent cell functions.