SUMMARY: The ability of washed suspensions of Y44 (which requires histidine and -aminobenzoic acid for growth) to synthesize methionine from homocysteine was decreased almost to zero when the organism was harvested from a growth medium containing methionine itself. There was a similar effect, though in no case quite so severe, with eleven other strains of the organism. In further experiments with strain Y 44 the effect was found to be specific to methionine; the presence during growth of no other amino acid tested decreased subsequent synthesis by more than 20%. Ability to synthesize methionine was lost and regained on single successive cultivations in the presence and absence of the amino acid; there was also no evidence from viable counts on agar media ± methionine for a genetic change. ‘Inactive’ organisms (methionine-grown) were not activated by changing the nature of the precursor of the methyl group nor by adding heated extracts of yeast or of ‘active’ organisms. There was no evidence that ‘inactive’ organisms produced an inhibitor of methionine synthesis. It is concluded that growth with methionine leads to the non-production of an enzyme(s) concerned directly in the addition of the one-carbon unit to homocysteine.


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