The CydDC ABC transporter of Escherichia coli: new roles for a reductant efflux pump

CydDC is an ATP-binding Cassette (ABC) transporter found within Escherichia coli (E. coli), previously demonstrated to be important for the maintenance of the redox poise of the periplasm through the active transport of cysteine and glutathione. cydDC knockout strains have previously been reported to be unable to assemble bd-type respiratory oxidases, lack motility, and are sensitive to benzylpenicillin and nitrosative stress. Previous work has identified a unique haem bound to the heterodimeric CydDC complex, hypothesised to bind to CydD at amino acid residues Arginine 47 and Histidine 51. In the current work, UV spectroscopic analysis of purified CydDC confirmed the presence of bound haem and tryptophan fluorescence titrations revealed a high affinity of CydDC for haem with a Kd of 0.32 µM. A CydDC expression vector was modified to encode a H51A variant of CydDC, but this mutation resulted in loss of assembly of the CydDC complex, indicating an important role for Histidine 51 of CydD in the complex assembly. Finally, disk diffusion assays were consistent with a role of CydDC in exporting glutathione:metal conjugates, reflecting a potentially novel role for CydDC in metal tolerance.