Synthesis from sulphate and accumulation of S-sulphocysteine by a mutant strain of Aspergillus nidulans

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Abstract
When the vegetative mycelia of a cysteine-less mutant, strain 793, of A. nidulans were aerobically incubated with a medium containing [S35]sulfate, unlabelled choline sulfate and DL-serine, practically all of the radioactivity was incorporated into the mycelia. The mycelia, after pretreatment with N-ethylmaleimide, were extracted and the extract was analyzed for radioactive metabolites by paper chromatography. Labelled S-sulfocysteine was detected, in addition to inorganic and choline sulfates and an unidentified S compound. The sulfocysteine was purified by column chromatography and its identity was confirmed by paper chromatography in 5 different solvent systems. The disulfide bond in the purified S-sulfocysteine was cleaved reductively with hydrogen sulfide or oxidatively with performic acid. By measuring the radio-activity of the cleavage products separately, it was concluded that the 2 S atoms of the accumulated sulfocysteine are equally labelled. Mutant strains with genetical blocking at earlier steps than S-sulfocysteine in the sulfate-assimilation pathway did not accumulate this compound when similarly incubated in the presence of [S35]sulfate. The mutants accumulated labelled inorganic sulfate and choline sulfate, except for a sulfite-less mutant, strain 954, which accumulated only inorganic sulfate. A trace of labelled S-sulfocysteine, together with relatively large amounts of inorganic and choline sulfates and an unidentified S compound, was detected in the extract of wild-type strain grown in the presence of [S35]sulfate. These findings have been discussed and it has been concluded that the pathway of sulfate assimilation in A. nidulans involves S-sulfocysteine as an obligatory intermediate.