The absence of a tryptophan-niacin relationship in Escherichia coli and Bacillus subtilis.

Nutritional studies with mammals led to the first suggestion of a relationship between tryptophan and niacin (Krehl et al., 1945). Subsequent investigations have established tryptophan as a precursor of niacin in many organisms and have led to the identification of several of the intermediates in this biosynthetic pathway (for reviews, see Dalgliesh, 1951; Bonner and Yanofsky, 1951). Direct demonstrations of the conversion of tryptophan to niacin have been provided by tracer studies performed with mammals (Heidelberger et al., 1949) and the mold Neurospora crassa (Partridge et al., 1952). In the latter case it was also shown that in this organism all niacin synthesis proceeds from tryptophan. Comparatively little is known about niacin synthesis in bacteria. Marnay (1951) has concluded, on the basis of inhibition studies, that the pathway of niacin synthesis in Escherichia coli is the same as in Neurospora. Ellinger and Abdel Kader (1949b) have also investigated niacin synthesis in E. coli and have presented evidence suggesting that tryptophan does not serve as a niacin precursor but functions catalytically in its formation. Several bacterial species can carry out a key sequence in niacin synthesis from tryptophan, the conversion of tryptophan to kynurenine (Stanier et al., 1951; Tabone and Robert, 1952). The conversion of kynurenine to niacin, however, has only ben shown in the bacterium Xanthoonas pruni (Davis et al., 1951). This organism also appears to be capable of forming niacin from tryptophan. During investigations with niacin auxotrophs of E. coli and Bacilus subtilis it became apparent that intermediates in niacin synthesis in neurospora would not support the growth of the bacterial mutants. This observation and others