The aromatic dioxygenase system in Sphingomonas yanoikuyae strain B1 consists of three components, an oxygenase, a ferredoxin, and a reductase. The insertional knockout of the bphA4 gene encoding a reductase and subsequent complementation experiments showed that the reductase encoded by bphA4 in S. yanoikuyae strain B1 is associated with multiple dioxygenase components including that of toluate dioxygenase (XylXY).