A mutant of Pseudomonas aeruginosa lacking the cytochrome c-551 gene (nirM) was constructed by marker exchange mutagenesis. When the nirM gene was introduced in this strain using a multi-copy expression vector, pHA10, recombinant cytochrome c-551 was produced as a hole-form under aerobic conditions, whereas the expression was not observed when the nirM gene specifically lacking the signal sequence for membrane translocation was used.