OXYGEN, IRON, CARBON, AND SUPEROXIDE CONTROL OF THE FUMARASE FUMA ANDFUMC GENES OF ESCHERICHIA-COLI - ROLE OF THE ARCA, FNR, AND SOXR GENE-PRODUCTS

The tricarboxylic acid cycle enzyme fumarase catalyzes the interconver sion of fumarate to L-malate. Escherichia coli contains three biochemi cally distinct fumarases. While the fumA and fumB genes encode heat-la bile, iron-containing fumarases, the fumC gene product is a heat-stabl e fumarase which does not require iron for activity. To study how the fumA and fumC genes are regulated, we constructed lacZ operon fusions to the fumA and/or fumC upstream regions. Expression of the fumA and f umC genes was lowest during anaerobic cell growth, in support of the p roposed roles of FumA and FumC as aerobic fumarases. Transcription of the fumC gene was shown to be complex: it was dependent on both the fu mA and fumC promoters. Anaerobic expression from the fumA promoter was derepressed in both an arcA and a fnr mutant, while expression from t he fumC promoter was derepressed in only the arcA strain. The fumA pro moter was also shown to be catabotite controlled, whereas the fumC pro moter was relatively unaffected by the type of carbon used for cell gr owth. Cellular iron limitation stimulated fumC but not fumA expression . Superoxide radicals also caused increased fumC gene expression; fumA expression was unaffected. Both the superoxide control and the iron c ontrol of fumC expression required the SoxR regulatory protein. These studies suggest different physiological roles for the FumA and FumC fu marases. The iron-containing FumA fumarase is the more abundant enzyme under most conditions of aerobic cell growth except when iron is limi ting; FumC, which lacks iron, appears to be a backup enzyme that is sy nthesized optimally only when iron is low or when superoxide radicals accumulate.