This disclosure provides an E. coli strain, which lacks thioredoxin reductase activity encoded by trxB and thioredoxin 1 activity encoded by trxA, and glutathione reductase activity encoded by gor. Said E. coli strain expresses a mutated AhpC protein having glutathione reductase activity and a cytosolic prokaryotic disulfide isomerase. The E. coli strain has an oxidative cytosol and can be used to efficiently produce proteins having disulfide bonds.