Wilson, R. G. (Oklahoma State University, Stillwater) and L. M. Henderson. Tryptophan-niacin relationship in Xanthomonas pruni. J. Bacteriol. 85:221-229. 1963.-The observation that Xanthomonas pruni, a bacterial pathogen for the peach, requires niacin for growth and can use tryptophan or 3-hydroxyanthranilic acid as a substitute was confirmed. To determine whether niacin is synthesized via the tryptophan-3-hydroxyanthranilic acid pathway, experiments using labeled metabolites were undertaken. Labeled tryptophan, 3-hydroxyanthranilic acid, quinolinic acid, and nicotinic acid were supplied in the basal medium in amounts sufficient to insure maximal growth. Nicotinic and quinolinic acids were isolated from the cells after the growth period. The isotope was incorporated from the first three labeled compounds into niacin with dilutions approximately the same in all cases, ranging from 7.6 to 17.1. The dilution of isotopic niacin was 3.1- to 5.9-fold. Only labeled quinolinic acid gave rise to labeled quinolinic acid in the cell, but this acid gave rise to niacin with 10- to 12-fold reduction in specific activity. The results indicate that if quinolinate participates as an obligatory intermediate in the synthesis of niacin from tryptophan, its concentration within the cell is very small and it does not equilibrate readily with exogenous quinolinate. The results confirm the conclusion, drawn from growth studies, that niacin is needed to permit tryptophan synthesis at a sufficient rate to promote growth. In the absence of an external source of niacin, tryptophan or some of its metabolites can promote growth by acting as precursors of niacin.