Metabolism of carbon monoxide by Rhodopseudomonas gelatinosa: cell growth and properties of the oxidation system.

Rhodopseudomonas gelatinosa 1 grew as an anaerobic facultative methylotroph with carbon monoxide as the sole carbon and energy source. Carbon from CO was assimilated into cell material via the ribulose 1,5-bisphosphate carboxylase cycle. The CO oxidation system in R. gelatinosa was induced during growth with the gas substrate. Light-grown cells did not oxidize CO. Surprisingly, when strain 1 cells grown in the dark with CO were transferred to growth with both CO and light, they continued to use CO and then photometabolized after the CO gas flow was stopped. This change in the energy-yielding substrate resulted in a diauxic growth response. The use of CO in preference to light energy forms the basis of a system in the cells that controls photosynthetic differentiation. CO oxidation was assayed as CO-methyl viologen oxidoreductase. Methyl viologen reduction only occurred with CO; the dye was not reduced with other C1 compounds. In vitro methyl viologen was reduced best at 24 degrees C and at pH values above 8.5. Whole cells exhibited a Km of 12.5 microM for CO and a Vmax of 3,800 nmol of CO oxidized per mg of protein per min. This was a low-potential oxidation reaction that readily reduced the viologen dye triquat (1,1'-trimethylene-2,2'-dipyridilium dibromide) (E degrees' = -548 mV).