[Light dependent carotenoid synthesis : V. Extinction of the photoinduction by reducing substances and substitution of hydrogen peroxide for light].

The effect of dithionite, hydroxylamine and hydrogen peroxide on the light dependent carotenoid synthesis in Fusarium aquaeductuum has been investigated in order to find indications whether redox reactions are involved in the first steps of photoinduction.Addition of dithionite (5·10(-3) M/l) to the mycelium some time after illumination prevented carotenoid synthesis completely; however, when dithionite was removed after 30 min by washing the mycelium with buffer, Fusarium synthesized nearly the same amounts of carotenoids as it does without dithionite incubation. To prevent this direct effect on biosynthesis of pigments, the mycelium was treated for only 30 min at different times before and after a short illumination with buffered dithionite solution. When dithionite was present during the illumination or was applied up to 21/2 min after the lights had been switched off, no carotenoids were synthesized at all. The inhibitory effect of dithionite gradually decreased during a 171/2 min period following the end of the illumination time. After this period treatment with dithionite showed no irreversible influence whatsoever on the carotenoid synthesis. Essentially the same results were obtained when hydroxylamine (10(-2) M/l, freshly prepared) was used as a reducing agent.On the other hand incubation with buffered hydrogen peroxide solution (10(-2) to 10(-1) M/l) in the dark simulated the effect of illumination in inducing carotenoid synthesis. Both the kinetics of the pigment production and the inhibition by cycloheximide suggest that treatment with hydrogen peroxide in the dark truly substitutes for photoinduction. From these results it is concluded that dithionite and hydroxylamine are capable of reducing as yet unknown "photooxidation products" which are produced during illumination, as proposed by several authors. This oxidative action of light can be simulated by incubation of the mycelium with hydrogen peroxide.Furthermore results are presented which suggest that in Fusarium light acts in two ways: 1. it induces a de novo protein synthesis giving rise to an enhanced carotenoid production ("light dependent synthesis") and 2. it inhibits a carotenoid synthesizing system ("dark synthesis") which functions with low activity in the mycelium in the dark.