Nitric oxide-induced bacteriostasis and modification of iron-sulphur proteins in Escherichia coli

The nitric oxide (NO) cytotoxicity has been well documented in bacteria and mammalian cells. However, the underlying mechanism is still not fully understood. Here we report that transient NO exposure effectively inhibits cell growth of Escherichia coli in minimal medium under anaerobic growth conditions and that cell growth is restored when the NO-exposed cells are either supplemented with the branched-chain amino acids (BCAA) anaerobically or returned to aerobic growth conditions. The enzyme activity measurements show that dihydroxyacid dehydratase (IlvD), an iron-sulphur enzyme essential for the BCAA biosynthesis, is completely inactivated in cells by NO with the concomitant formation of the IlvD-bound dinitrosyl iron complex (DNIC). Fractionation of the cell extracts prepared from the NO-exposed cells reveals that a large number of different protein-bound DNICs are formed by NO. While the IlvD-bound DNIC and other protein-bound DNICs are stable in cells under anaerobic growth conditions, they are efficiently repaired under aerobic growth conditions even without new protein synthesis. Additional studies indicate that L-cysteine may have an important role in repairing the NO-modified iron-sulphur proteins in aerobically growing E. coli cells. The results suggest that cellular deficiency to repair the NO-modified iron-sulphur proteins may directly contribute to the NO-induced bacteriostasis under anaerobic conditions.