THE IRREVERSIBILITY OF METHIONINE SYNTHESIS FROM CYSTEINE IN PASTEURELLA PESTIS1

In a study of the nutrition of Pasteurella pestis, strain A1122, it was found that this organism requires both thiosulfate and methionine for growth. In a similar investigation of both virulent and avirulent strains of P. pestis, Hills and Spurr (1950) and Rockenmacher (1950) made the observation that both cystine and methionine are essential. This natural requirement for two sulfur-containing compounds by several strains of P. pestis and the indication that the cysteine-to-methionine series of reactions (Horowitz, 1947) would not be compli-cated by the presence of an alternate pathway for the transfer of the sulfur of methionine to cysteine, as seems to be the case in mutants of Neurospora (Horowitz, 1947) and Escherichia coli, strain 15 (Lampen et al., 1947), prompted a more detailed exploration of the sulfur metabolism of this organism. MATERIALS AND METHODS Pasteurella pestis, strain A1122, avirulent, originally isolated from a virulent strain by Jawetz and Meyer (1943) was employed in this study. Stock cultures were maintained originally on blood hormone agar and subs