LIGHT-INDUCED REACTIONS OF UBIQUINONE IN PHOTOSYNTHETIC BACTERIUM, CHROMATIUM D

Light-induced changes in ultraviolet absorption were investigatedin a photosynthetic bacterium, Chromatium strain D. The difference spectra (light-minus-dark) of the changes insteady state level of absorption under various experimentalconditions have common maxima (peaks or troughs) at 275 mµ,315 mµ and 340 mµ; The sign of change, however,varied according to the conditions of illumination (see below).From the shape of the difference spectra and for other reasonsdiscussed in the present paper, the changes at 275 mµwere ascribed to the oxidation-reduction changes of ubiquinone. Illumination under aerobic condition caused a reduction of ubiquinonsamounting to about 6–7 percent of total ubiquinone inthe cell. The addition of Na₂S₂O₃ enhanced the amplitude ofthe photoinduced change to about 25% of the total ubiquinonein the cell. The sign of the photoinduced absorption change at 275 mµwas reverted by the presence of malate or succinate as substrate.On illumination under anaerobic condition, there was a photoinducedoxidation of ubiquinone amounting to about 50% of the total.Under aerobic condition the amount was about 6–7 percentof the total. Transitional changes of ubiquinone in the bacterial cell werealso investigated under various experimental conditions. (Received July 24, 1967; )