Sulfite dehydrogenases, which oxidize sulfite to sulfate, are best known within the major fluxes of the sulfur cycle catalyzed during chemolithotrophic and phototrophic growth of e.g. Paracoccus pantotrophus, Starkeya novella and Chlorobium tepidum. Somewhat smaller, organotrophic fluxes of organosulfonates are also part of the sulfur cycle, in which sulfite from the dissimilation of methansulfonate, dimethylsulfoxide, taurine (and derivatives), sulfolactate and sulfoquinovose, as well as from xenobiotic sources, is oxidized to sulfate. Evidence now indicates that a novel, inducible sulfite dehydrogenase with an unknown native electron acceptor is involved in many organisms. We have established that the enzyme from Cupriavidus necator H16 (previously Ralstonia eutropha), whose genome sequence is available, can be assayed with ferricyanide as the electron acceptor and separated by column chromatography. We will purify and characterize the enzyme, and thus identify the gene and its regulator. Another target is the nature of the native electron acceptor. Strain H16 potentially encodes several orthologues of known sulfite dehydrogenases, whose role in organotrophic metabolism will also be explored.