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Tryptophan Biosynthesis from Indole-3-Acetic Acid by Anaerobic Bacteria from the Rumen

¹ National Animal Disease Laboratory, Agricultural Research Service, Ames, Iowa 50010

ABSTRACT

Microbes in ruminal contents incorporated ¹⁴C into cells when they were incubated in vitro in the presence of [¹⁴C]carboxyl-labeled indole-3-acetic acid (IAA). Most of the cellular ¹⁴C was found to be in tryptophan from the protein fractions of the cells. Pure cultures of several important ruminal species did not incorporate labeled IAA, but all four strains of Ruminococcus albus tested utilized IAA for tryptophan synthesis. R. albus did not incorporate ¹⁴C into tryptophan during growth in medium containing either labeled serine or labeled shikimic acid. The mechanism of tryptophan biosynthesis from IAA is not known but appears to be different from any described biosynthetic pathway. We propose that a reductive carboxylation, perhaps involving a low-potential electron donor such as ferredoxin, is involved.