SUMMARY: Mutant strains of requiring methionine for growth were isolated and their growth responses to inorganic and organic sulphur sources studied. It is suggested that methionine is synthesized in this mould from inorganic sulphate through sulphite, thiosulphate and cysteine. The mutant strains genetically blocked in the reaction from thiosulphate to cysteine could be divided into two genetically different groups by means of the heterokaryosis test; heterokaryons formed between these two groups were able to grow on sulphate as sole sulphur source because of the syntrophic action of two different types of nuclei. It was further shown that cysteine-S-sulphonate supported good growth of one of the two groups, whereas the other group showed no growth on this compound. It was concluded that the metabolic conversion of thiosulphate to cysteine involves cysteine-S-sulphonate as an intermediate. Mutant strains responded to sulphide in the same way as to thiosulphate. From this and other evidence, it is suggested that sulphide is utilized by way of thiosulphate.


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