[Light dependent carotenoid synthesis : VIII. Different action mechanisms of light and mercuribenzoate]

The fungus Fusarium aquaeductuum synthesizes large amounts of carotenoids only after illumination or in the presence of mercuribenzoate (HMB) in the dark. The effect of HMB is abolished entirely by the addition of excess thiols such as cysteine or mercaptoethanol; evidently "active" HMB must be present continuously within the cell. In contrast, photoinduction needs only a short exposure to light to set off pigment production in a subsequent dark period. Illumination of mycelia preincubated with HMB in the dark induces an additional proportion of carotenoid synthesis which shows the same kinetics and quantitative yields as that observed in untreated controls. This demonstrates that HMB, though inhibiting O2-uptake by 50%, does not interfere with light-induced carotenoid production as such. The effects of light and HMB are additive under various experimental conditions tested. After addition of thiols to mycelia treated with HMB in the light, carotenoid production decreased to an extent equal to the amount mediated by HMB in the dark controls. These results provide strong evidence that the mechanisms and the action sites of light and HMB in carotenoid synthesis are different. Incubation with HMB after exposure to light reduces the agent's effectiveness, which declines gradually to about 30% during the first two hours.These findings are interpreted to mean that in Fusarium there are two isoenzyme systems at work in carotenoid production: a constitutive one showing very low net activity is subject to direct stimulation by HMB; light somehow eliminates this enzyme system and, at the same time, induces an isoenzyme set which exhibits high carotenogenic activity and is insensitive to HMB treatment.