The CydDC complex of Escherichia coli is a heterodimeric ATP-binding cassette (ABC) transporter that exports cysteine and glutathione to the periplasm. These reductants are thought to modulate periplasmic redox poise, impacting upon the disulfide folding of periplasmic and secreted proteins involved in bacterial virulence. Diminished CydDC activity abolishes the assembly of functional bd-type respiratory oxidases and perturbs haem ligation during the assembly of c-type cytochromes. The focus herein is upon a newly-discovered interaction of the CydDC complex with a haem cofactor; haem has recently been shown to modulate CydDC activity and structural modelling reveals a potential haem-binding site on the periplasmic surface of the complex. These findings have important implications for future investigations into the potential roles for the CydDC-bound haem in redox sensing and tolerance to nitric oxide (NO).

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