Mutants of Escherichia coli lacking disulphide oxidoreductases DsbA and DsbB cannot synthesise an exogenous monohaem c-type cytochrome except in the presence of disulphide compounds.

Absence through mutation of two proteins involved in periplasmic disulphide bond formation, DsbA and DsbB, results in failure of anaerobically grown Escherichia coli to synthesise the holo forms of either its endogenous c-type cytochrome nitrite reductase or exogenous cytochrome c550 from Paracoccus denitrificans. The synthesis of both cytochromes can be restored to the mutants by inclusion in the growth media of compounds containing disulphide bonds, e.g., the oxidised form of glutathione. The results suggest that the attachment of haem to the CXXCH motif of a periplasmic c-type cytochrome may be preceeded by the formation of one or more intra- or intermolecular disulphide bonds involving the cysteine residues of this motif.