A UNIQUE FUNGAL LYSINE BIOSYNTHESIS ENZYME SHARES A COMMON ANCESTOR WITH TRICARBOXYLIC-ACID CYCLE AND LEUCINE BIOSYNTHETIC-ENZYMES FOUND INDIVERSE ORGANISMS

Fungi have evolved a unique alpha-aminoadipate pathway for lysine bios ynthesis. The fungal-specific enzyme homoaconitate hydratase from this pathway is moderately similar to the aconitase-family proteins from a diverse array of taxonomic groups, which have varying modes of obtain ing lysine. We have used the similarity of homoaconitate hydratase to isopropylmalate isomerase (serving in leucine biosynthesis), aconitase (from the tricarboxylic acid cycle), and iron-responsive element bind ing proteins (cytosolic aconitase) from fungi and other eukaryotes, eu bacteria, and archaea to evaluate possible evolutionary scenarios for the origin of this pathway. Refined sequence alignments show that acon itase active site residues are highly conserved in each of the enzymes , and intervening sequence sites are quite dissimilar. This pattern su ggests strong purifying selection has acted to preserve the aconitase active site residues for a common catalytic mechanism; numerous other substitutions occur due to adaptive evolution or simply lack of functi onal constraint. We hypothesize that the similarities are the remnants of an ancestral gene duplication, which may not have occurred within the fungal lineage, Maximum likelihood, neighbor joining, and maximum parsimony phylogenetic comparisons show that the alpha-aminoadipate pa thway enzyme is an outgroup to all aconitase family proteins for which sequence is currently available.