1. The pattern of distribution on the purine pathway of mutants of Salmonella typhimurium LT2 that had the double growth requirement for a purine plus the pyrimidine moiety of thiamine (ath mutants) indicated that purines and the pyrimidine moiety of thiamine share the early part of their biosynthetic pathways, and that 4-aminoimidazole ribonucleotide (AIR) is the last common intermediate. Two mutants that at first appeared anomalous were further investigated and found not to affect this deduction. 2. The ribonucleoside form of AIR (AIR(s)) satisfied the requirements both for a purine and for the pyrimidine moiety of thiamine of an ath mutant. 3. Methionine was required for the conversion of AIR into the pyrimidine moiety. 4. Radioactive AIR(s) was converted into radioactive pyrimidine moiety by an ath mutant without significant dilution of specific radioactivity. 5. Possible mechanisms for pyrimidine-moiety biosynthesis from AIR are discussed.

1. A method was devised for obtaining the pyrimidine moiety of thiamine in a pure form after its excretion into the medium by de-repressed washed-cell suspensions of mutants of Salmonella typhimurium LT2. 2. By using amino acid-requiring mutants, this excretion of pyrimidine moiety was shown to be dependent on the presence of both methionine and glycine. 3. In the presence of either [Me-(14)C]methionine or [G-(14)C]methionine, methionine-requiring mutants did not incorporate radioactivity into the pyrimidine moiety. 4. In contrast, both [1-(14)C]glycine and [2-(14)C]glycine were incorporated into the pyrimidine moiety excreted by glycine-requiring mutants, and this occurred with little or no dilution of specific radioactivity. 5. The possible requirement for methionine as a cofactor and the significance of the incorporation of both carbon atoms of glycine are discussed.

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1. Growth of Salmonella typhimurium LT2 in the presence of adenosine was shown to cause enormous synthesis of thiamine in washed-cell suspensions. 2. Evidence that this was due to de-repression and not an accumulation of precursors was obtained by using a mutant blocked in the biosynthesis of the thiazole moiety, which showed a similarly large synthesis of the pyrimidine of thiamine. 3. The specific requirements for a source of energy, nitrogen and sulphur were investigated, and indicated new synthesis in this system.

1. Thiamine or the pyrimidine moiety of thiamine added in excess to a growing culture of Salmonella typhimurium LT2 repressed subsequent thiamine synthesis in non-growing organisms. 2. A mutant unable to convert added pyrimidine moiety into thiamine was not repressible by the pyrimidine, showing that thiamine, not the pyrimidine, was the repressor. 3. Thiamine repression occurred at 40mmug. of thiamine/mg. dry wt. or above and de-repression occurred at 30mmug. of thiamine/mg. dry wt. or below. 4. Thiamine controlled the pyrimidine and thiazole pathways at the same concentration and to the same extent. 5. Biosynthesis of the thiazole moiety had, in contrast with biosynthesis of the pyrimidine moiety, an additional feedback inhibition control that allowed utilization of the exogenous thiazole. 6. The enzymes joining the pyrimidine and thiazole moieties were repressible by high concentrations of thiamine. 7. Thiamine was rapidly converted into thiamine pyrophosphate and this appeared to be the active repressor. 8. Theoretical aspects of control of converging pathways are discussed.