Convergent Evolution of Ureide-Catabolizing Enzymes in Plants and Animals.

Allantoicase and allantoate amidohydrolase (AAH) are important ureide-catabolizing enzymes in purine salvage.   Both enzymes break down allantoate to ureidoglycolate, however, allantoicase and AAH play different roles in animal and plant metabolism, respectively.  In lower vertebrates (i.e., amphibians and fish), allantoicase provides a means of removing nitrogen waste products via urea.  In contrast, plants use AAH to salvage nitrogen for re-incorporation into amino acids and nucleic acids.  This is especially important in legumes, such as soybean, where nitrogen fixed by Bradyrhizobium is transported from the nodules to the shoots via the xylem as the ureide, allantoate.  Recently, AAH was cloned from Arabidopsis and soybean.  Both plant AAH genes restore allantoate metabolism in a yeast mutant defective in allantoicase.  These results indicate the redundant function of the allantoicase and AAH in allantoate breakdown.  Therefore, we hypothesized that the two enzymes were homologous and originated from a single ancestral gene.  Expressed sequence tags and complete mRNA sequences for each gene available from GenBank were collected for bacteria, yeast, flowering plants, and lower vertebrates.  The sequences were evaluated for amino acid similarity and identity between the two enzymes using alignment software and BLAST.  As expected, genes for allantoicase were identified in yeast and animals and AAH from plants.  Although both genes were identified in bacteria, they were mutually exclusive among phyla.  Furthermore, no amino acid sequence similarity was detected between the allantoicase and AAH genes.  These results indicate the independent development or convergent evolution of both genes into a common pathway for purine salvage.