Physiological roles and adverse effects of the two cystine importers of Escherichia coli

When cystine is added to Escherichia coli, the bacterium becomes remarkably sensitive to hydrogen peroxide. This effect is due to enlarged intracellular pools of cysteine, which can drive Fenton chemistry. Genetic analysis linked the sensitivity to YdjN, a secondary transporter that along with the FliY-YecSC ABC system is responsible for cystine uptake. FliY-YecSC has a nanomolar K_m and is essential for import of trace cystine, whereas YdjN has a micromolar K_m and is the predominant importer when cystine is more abundant. Oddly, both systems are strongly induced by the CysB response to sulfur scarcity. The FliY-YecSC system can import a variety of biomolecules, including diaminopimelate; it is therefore vulnerable to competitive inhibition, presumably warranting YdjN induction under low-sulfur conditions. But the consequence is that if micromolar cystine then becomes available, the abundant YdjN massively overimports it, at >30 times the total sulfur demand of the cell. The imported cystine is rapidly reduced to cysteine in a glutathione-dependent process. This action avoids the hazard of disulfide stress, but it precludes feedback inhibition of YdjN by cystine. We conjecture that YdjN possesses no cysteine allosteric site because the isostructural amino acid serine might inappropriately bind in its place. Instead, the cell partially resolves the overaccumulation of cysteine byimmediately excreting it, completing a futile import/reduction/export cycle that consumes a large amount of cellular energy. These unique, wasteful, and dangerous features of cystine metabolism are reproduced by other bacteria. We propose to rename ydjN as tcyP and fliY-yecSC as tcyJLN.