5'-Adenylylsulfate (APS) reductase from Enteromorpha intestinalis (EiAPR) is composed of two domains that function together to reduce APS to sulfite. The carboxyl-terminal domain functions as a glutaredoxin that mediates the transfer of electrons from glutathione to the APS reduction site on the amino-terminal domain. To study the basis for the interdomain interaction, a heterologous system was constructed in which the C domain of EiAPR was fused to the carboxyl terminus of the APS reductase from Pseudomonas aeruginosa (PaAPR), an enzyme that normally uses thioredoxin as an electron donor and is incapable of using glutathione for this function. The hybrid enzyme, which retains the [4Fe-4S] cluster from PaAPR, was found to use both thioredoxin and glutathione as an electron donor for APS reduction. The ability to use glutathione was enhanced by the addition of Na2SO4 to the reaction buffer, a property that the hybrid enzyme shares with EiAPR. When the C domain was added as a separate component, it was much less efficient in conferring PaAPR with the ability to use glutathione as an electron donor, despite the fact that the separately expressed C domain functioned in two activities that are typical for glutaredoxins, hydroxyethyl disulfide reduction and electron donation to ribonucleotide reductase. These results suggest that the physical connection of the reductase and C domain on a single polypeptide is critical for the electron-transfer reaction. Moreover, the effect of Na2SO4 suggests that a water-ordering component of the reaction milieu is critical for the catalytic function of plant-type APS reductases by promoting the interdomain interaction.