The process of carotenoid biosynthesis in C. diospyri was found to have an absolute requirement for light, and only light of a wavelength less than 500nm was effective for this photoinduction. An analysis of the carotenoid pigments of C.diospyri revealed that only phytoene was present in detectable amounts in dark grown cells. This might suggest that light was responsible for the expression of a hypothetical enzyme, phytoene dehydrogenase, which by a process of dehydrogenation was responsible for the appearance of the coloured carotenoids present in cells exposed to light. A study of the mechanism of the photoinduction of carotenoids would indicate a process very similar to those described for N.crassa (Zalokar, 1954, 1955), Mycobacterium sp. (Rilling, 1961, 1964) and F.aquaeductuum (Eberhard et al, 1961), except that the production of carotenoids appeared to proceed at a much slower rate in C.diospyri. Three separate stages were recognisable. The initial photoinduction which, unlike the remainder of the process required light, was independent of temperature and required oxygen. The subsequent dark incubation consisted of a lag period before any carotenoids were synthesised followed by the actual production of the pigments. Both these latter stages were temperature dependent and also required oxygen. Carotenoid synthesis in C.diospyri was inhibited by both diphenylamine and cyclohexamide, and only the stages after the initial photoinduction were sensitive to diphenylamine. Both PCMB and PHMB were found to substitute for light in carotenoid production in C.diospyri, but only when the fungus was grown on solid medium. No other -SH group inhibitor tested behaved as a photomimetic compound. In addition, antimycin A, found by Batra (1967) to have a similar property with Mycobacterium marinum was ineffective against C.diospyri except that at higher concentrations inhibited both growth and carotenoid production. Filtrates from dark grown cultures of C.diospyri were found to contain riboflavin with the corresponding production of lumichrome its acidic photolytic degradation product, in filtrates from light grown cultures. The physiology of the biosynthesis of riboflavin by C.diospyri appeared to resemble the process described by Kapralek (1962) for Eremothecium ashbyii, where cytochrome mediated respiration was replaced by a flavin type of respiration. Diphenylamine, but not cyclohexamide, in the same range of concentrations which inhibited carotenoid synthesis in light grown cells of C.diospyri also inhibited the extracellular production of riboflavin by dark grown cells. No conclusive evidence was obtained which might suggest that light was responsible for the alteration of any other aspect of the metabolism of C.diospyri. Results would suggest that, in many areas of the metabolism, light and dark grown cells were the same. Parameters studied included sterol content of cells, the composition of their amino acid pools, their organic acid content, keto acids and soluble protein and isoenzyme composition. The results obtained in this investigation are discussed in the light of other work and suggestions for possible lines of future research proposed.
|Date of Award||1970|