One of the last unsolved problems of molecular biology is how the sequential amino acid information leads to a functional protein. Correct disulfide formation within a protein is hereby essential. We present periplasmic ribonuclease I (RNase I) from Escherichia coli as a new endogenous substrate for the study of oxidative protein folding. One of its four disulfides is between nonconsecutive cysteines. In general view, the folding of proteins with nonconsecutive disulfides requires the protein disulfide isomerase DsbC. In contrast, our study with RNase I shows that DsbA is a sufficient catalyst for correct disulfide formation in vivo and in vitro. DsbA is therefore more specific than generally assumed. Further, we show that the redox potential of the periplasm depends on the presence of glutathione and the Dsb proteins to maintain it at-165 mV. We determined the influence of this redox potential on the folding of RNase I. Under the more oxidizing conditions of dsb(-) strains, DsbC becomes necessary to correct non-native disulfides, but it cannot substitute for DsbA. Altogether, DsbA folds a protein with a nonconsecutive disulfide as long as no incorrect disulfides are formed.