Metabolism of elemental sulphur (S°) during fungal spore germination

Abstract

35S-labelled elemental sulphur (S°) at nontoxic levels (3 μM) was incorporated into sulphur amino acids and glutathione by the spores of the fungus Phomopsis viticola. Incorporation studies were performed with 3 μM 35S° during the pregermination period of P. viticola spores. The free sulphur amino acids and protein sulphur amino acids were purified by column and thin-layer chromatography. During the first minutes of the pregermination process of the spores, the S° was essentially metabolized into free cysteine and glutathione. Detectable concentrations of methionine and homocysteine were measured after 1 h of incubation. Azide (2 mM final concentration), an inhibitor of the mitochondrial respiratory chain at the level of cytochrome oxidase, strongly inhibited the incorporation of S°. In contrast, the uncoupler 2,4-dinitrophenol (15 μM final concentration) and sulphate did not affect the incorporation of S°. The metabolism of 35S° into the protein sulphur amino acids cysteine and methionine during the germination of spores of P. viticola and conidia of Neurospora crassa is also discussed. It appears that S° does not necessarily have to be oxidized to sulphate prior to its incorporation into sulphur amino acids, but could be directly reduced to sulphide at the level of the respiratory chain.Key words: elemental sulphur, assimilation, sulphur amino acids, Phomopsis viticola, Neurospora crassa.