Photosensitivity of the respiratory chain in Neurospora crassen

Abstract

The photosensitivity of respiration in a carotenoid synthesising strain (FGSC 987) and a carotenoidless mutant (FGSC 16) of Neurospora crassa has been studied. Although I have failed to find any effect of light on growth, violet light (385--495 nm, 150 W/m2) inhibits respiration in both strains. However, mycelia of the yellow strain grown in the light have a high carotenoid content and show a smaller inhibition of respiration than those of the white or the dark-grown yellow strain.

Respiration of isolated mitochondria from yellow and white strains are also inhibited by violet light. Three photosensitive sites have been identified in the succinoxidase of both strains. These are: 1. A photosensitive site identified as ubiquinone; 2. A sensitive site repaired by treatment with thiol reagents such as cysteine, glutathione and dithiothreitol; 3. A weakly sensitive site associated with succinate dehydrogenase assayed with phenazine methosulphate as electron acceptor. The cytochrome oxidase is not light sensitive unless given prior treatment with a lipid-oxidising enzyme (lipoxygenase).

The NADH-oxidising pathway was less photosensitive than succinoxidase. The photosensitivity of a sulpkydryl group and ubiquinone have been demonstrated in the NADH pathway.

Growth of the pigmented strain in light and good aeration produce hyphae with a high carotenoid content. Ubiquinone in the mitochondria from these mycelia is much less photosensitive when irradiated with yellow light (440 + nm, 532 W/m2) than is the ubiquinone of mitochondria from the white strain or the dark-grown yellow strain. Mitochondria from the yellow strain are found to contain carotenoid. In dark-growncultures this pigment is located in the outer membrane, while in light-grown cultures the pigment is found in both mitochondrial membranes. It is concluded that in cultures grown in the light, carotenoids protect ubiquinone from photodestruction when they are located in the inner membrane.

Carotenoids, even at low levels, seemed to protect kynurenine hydroxylase, the outer membrane enzyme, against the effect of violet light. Protection of this enzyme against H₂O₂ has also been demonstrated but this requires high levels of carotenoid.