STRUCTURE AND FUNCTION OF THE TETRAHEME CYTOCHROME ASSOCIATED TO THE REACTION-CENTER OF ROSEOBACTER-DENITRIFICANS

We have characterized the tetrahemic RC bound cytochrome isolated from the quasi-photosynthetic bacterium Roseobacter denitrificans in terms of absorption spectrum, redox property and orientation with respect to the membrane plane. The heme, designated H-1, which possesses the highest redox midpoint potential (+290 mV), absorbs at 555 nm. Its plane makes an angle of 40 degrees with the membrane plane. The second high potential heme, H-2 (+240 mV), peaks at 554 nm and makes a tilt of 55 degrees with the membrane. The two low potential hemes, L(1) and L(2), present a similar and rather high redox midpoint potential (+90 mV). They absorb at 553 nm and 550 nm. One of these hemes is oriented at 40 degrees while the other makes an angle of 90 degrees with the membrane plane. The soluble cytochrome c(551) completes the cyclic electron transfer between the RC and the be, complex. Both the oxidation and the rereduction of cytochrome c(551) are diffusible processes. Under semi-aerobic conditions, one of the low potential hemes is photo-oxidized under illumination but only extremely slowly re-reduced. This explains the requirement of high aerobic conditions for growth of Roseobacter denitrificans cells in the light.