The influence of folic acid and vitamin B12 on nucleic acid metabolism in microorganisms.

A combination of purines and thymine has long been known to substitute for folic acid (FA) in the nutrition of microorganisms requiring the vitamin. The purines were found to be interchangeable, but the effect of thymine was specific and could only be duplicated by its desoxyriboside, thymidine (l-4). From these and other studies on inhibition analysis (5, S), it was concluded that the purines and thymine are metabolic endproducts of reactions mediated by FA. Prusoff et al. (7) compared the nucleic acid make-up of Lactobacillus casei cells grown with optimal and suboptimal concentrations of FA and observed that the desoxypentose nucleic acid (DNA) content was depressed markedly in the deficient cells, whereas the level of pentose nucleic acid (PNA) was unaltered. Growth studies with microorganisms requiring vitamin Blz showed that the vitamin could be replaced by purines and thymidine. The desoxyribosides and desoxyribotides of purines and of cytosine could also support partial growth (8-11). Mediation of vitamin Blz in nucleic acid synthesis was further shown by Roberts et al. (12). These and other functional relationships between FA and vitamin Blz suggested an investigation of their effects on nucleic acid metabolism and are contained in part in a preliminary communication (13). The organisms chosen were L. casei 7469, and Streptococcus faecalis R 8043 requiring FA, Lactobacillus leichmannii 313 requiring vitamin B12, as well as, reportedly (14), FA, and La&bacillus arabinosus 8014 and Escherichia coli McLeod, neither of which requires either vitamin for normal growth.