THE IRREVERSIBILITY OF METHIONINE SYNTHESIS FROM CYSTEINE IN PASTEURELLA PESTIS

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Abstract
P. pestis, strain A1122, requires as N and S sources for growth DL-phenylalanine, DL-valine, DL-isoleucine, DL-methionine, and thiosulfate. In the presence of DL-methionine, thiosulfate in replaceable only by L-cysteine, sulfide, or sulfite, and in the presence of thiosulfate, DL-methionine is replaceable only by DL-homocysteine or L-cystathionine. This dual S requirement is explained by the inability of this organism to synthesize cysteine and methionine from inorganic sulfate by virtue of breaks probably in the conversion of sulfate to sulfite and of cysteine to cystathionine and the lack of an alternate pathway for converting methionine S to cysteine S. A "spontaneous" mutant was isolated which utilizes sulfite, thiosulfate, sulfide, or L-cysteine as sole S source, indicating that a gain mutation probably occurred at the systeine-to-cystathionine site, thus enabling this mutant to synthesize methionine from any of the above four compounds. The fact that this mutant is still unable to use DL- methionine, DL-homocysteine, or L-cystathionine as sole S source demonstrates the irreversibility of the cysteine-to-methionine synthesis in this organism.