Lane 1, wt DsrC dimeric, Lane 2, DsrC-Cys100Ser dimeric, Lane 3, DsrC-Cys111Ser monomeric, Lane 4 DsrC-Cys100Ser/Cys111Ser monomeric (a) proteins as isolated (b) proteins pretreated with iodoacetamide leading to alkylation of free cysteine residues (c) proteins reduced with 2-mercaptoethanol after treatment with iodoacetamide