Multiple pathways for isoleucine biosynthesis in the spirochete Leptospira

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Abstract
Spirochetes of the genus Leptospira were previously shown to use an unusual pathway to synthesize isoleucine. For reasons of convenience, it is assumed that only 1 unusual pathway is found in the genus and it is referred to it as the pyruvate pathway. The distribution of this pyruvate pathway was determined in representatives of the 7 Leptospira DNA hybridization groups. The method included labeling the representative strains with radioactive CO2 and other radioactive precursors, fractionating the cells and determining the specific activities (counts detected/nmol) of the amino acids found in the protein fractions. On the basis of isoleucine biosynthesis, it was found that the genus can be classified as follows: class I primarily, if not exclusively, uses the well-known threonine pathway; class II uses mostly the pyruvate pathway, with a minor amount of isoleucine being synthesized via the threonine pathway; and class III uses the pyruvate pathway exclusively. No relationship appears to exist between the degree of DNA hybridization and the classes of isoleucine biosynthesis. Although the precise intermediates on the pyruvate pathway are unknown, the origin of the C skeleton of isoleucine synthesized by this pathway is consistent with a borrowing of the leucine biosynthetic enzymes. The pyruvate pathway is not controlled by leucine and that the 2 isoleucine pathways are independently regulated. Finding major and highly evolved multiple biosynthetic pathways of a specific amino acid within 1 genus is unique, and, conceivably, represents phylogenetic diversity within Leptospira.