Horizontal gene transfer from Eukarya to bacteria and domain shuffling: the alpha-amylase model.

Alpha-amylases are present in all kingdoms of the living world. Despite strong conservation of the tertiary structure, only a few amino acids are conserved in interkingdom comparisons. Animal alpha-amylases are characterized by several typical motifs and biochemical properties. A few cases of such alpha-amylases have been previously reported in some eubacterial species. We screened the bacterial genomes available in the sequence databases for new occurrences of animal-like alpha-amylases. Three novel cases were found, which belong to unrelated bacterial phyla: Chloroflexus aurantiacus, Microbulbifer degradans, and Thermobifida fusca. All the animal-like alpha-amylases in Bacteria probably result from repeated horizontal gene transfer from animals. The M. degradans genome also contains bacterial-type and plant-type alpha-amylases in addition to the animal-type one. Thus, this species exhibits alpha-amylases of animal, plant, and bacterial origins. Moreover, the similarities in the extra C-terminal domains (different from both the alpha-amylase domain C and the starch-binding domain), when present, also suggest interkingdom as well as intragenomic shuffling.