1. Caesium(1+) is a Caesium ion, a monovalent inorganic cation, a monoatomic monocation and an alkali metal cation

2. An E. coli strain containing a ferredoxin gene originating in actinomycete, a ferredoxin gene foreign to E. coli and a ferredoxin reductase gene, which is useful in the progress of an effective reaction of adding monoatomic oxygen to an organic compound serving as a substrate, is provided.