Increases in Cellular Levels of Cytochrome cd1 in Roseobacter denitrificans upon Irradiation with Green Light during Aerobic Growth

The cellular level of cytochrome cd1, the nitrite reductase of the aerobic photosynthetic bacterium Roseobacter denitrificans, increased considerably when the cells were grown aerobically under white light. The action spectrum for the increase, determined both spectroscopically and immunologically, revealed that green light at 561 nm was most effective, while blue light between 400 and 500 nm was fairly effective. Red and far-red light (650–900 nm) absorbed by the bacterio-chlorophyll had no effect, even though bacteriochlorophyll and carotenoids were formed normally during the growth of cells. Diphenylamine, an inhibitor of the biosynthesis of carotenoids abolished the increase in levels of the cytochrome, a result that suggests that a carotenoid(s) was responsible for this phenomenon. The bulk carotenoids seem, however, to be unlikely the candidates for the photoreceptors because they did not accumulate in the light-grown cells. Attempts to detect archaerhodopsin, 11-cis and all-trans retinal by immunological or HPLC analysis were unsuccessful. Although we failed to identify the photoreceptor, it is clear that R. denitrificans has a green-light signal-transduction system that controls the expression of cytochrome cd1.