The Role of Vitamin B,, in Thymine Biosynthesis by Lactobacillus leichmannii*

The biochemistry of vitamin Biz has been the subject of several recent reviews (1, 2, 3). Evidence currently available indicates that the vitamin is required for the reduction of formate to methionine and choline methyl. Indirect evidence implicates vitamin Biz in thymine biosynthesis. Thymine will replace the Blz requirements in certain microorganisms (3) and will induce a reticulocyte response in pernicious anemia patients (4). Bone marrow cells from pernicious anemia patients exhibit a reduced deoxyribonucleic acid to ribonucleic acid ratio which may reflect impaired thymine biosynthesis (5). The demonstration that formate may be reduced to thymine methyl (6, 7) suggests that Blz may influence thymine biosynthesis by catalyzing this reduction step. The present experiments were designed to test this hypothesis. The vitamin B12 requirement of Lactobacillus leichmannii may be satisfied by a number of deoxyribosides (8). Vitamin Blz-deficient cells were obtained by growing the organism with deoxycytidine. The results to be reported indicate that vitamin Bi2 is required for the reduction of formate to thymine methyl by this organism.