Abstract
1. 1. The regulation of NAD biosynthesis was studied in Escherichia coli, Bacillus subtilis and Bacillus megaterium, bacteria which utilize conventional metabolites and an “aspartate-glycerol pathway” for NAD biosynthesis and Xanthomonas pruni, the only bacterium in which tryptophan is known to serve as a precursor to NAD. The recent findings that in bacteria quinolinic acid occurs via the aspartate-glycerol pathway, suggested the need for a more recent evaluation. 2. 2. The biosynthesis of quinolinic acid via the aspartate-glycerol pathway in E. coli is repressed when cells are grown in the presence of excessive levels of nicotinic acid. In addition, nicotinate phosphoribosyltransferase (EC 2.4.2.11), an enzyme on the ‘salvage pathway’ for NAD biosynthesis, is repressed by comparable levels of nicotinic acid. However, quinolinate phosphoribosyltransferase is unaffected under the same conditions of growth. In contrast, in B. subtilis and B. megaterium quinolinate phosphoribosyltransferase is repressed by growth on increased levels of nicotinic acid, but nicotinate phosphoribosyltransferase is not. 3. 3. None of the enzymes of the tryptophan-NAD pathway studied in Xanthomonas pruni was repressed by growth in the presence of excess nicotinic acid. However, feedback inhibition of tryptophan pyrrolase may offer a means for the regulation of NAD biosynthesis in this organism.
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