Lateral gene transfer of a dermonecrotic toxin between spiders and bacteria

Motivation: Spiders in the genus Loxosceles, including the notoriously toxic brown recluse, cause severe necrotic skin lesions owing to the presence of a venom enzyme called sphingomyelinase D (SMaseD). This enzyme activity is unknown elsewhere in the animal kingdom but is shared with strains of pathogenic Corynebacteria that cause various illnesses in farm animals. The presence of the same toxic activity only in distantly related organisms poses an interesting and medically important question in molecular evolution. Results: We use superpositions of rece ntly determined structures and sequence comparisons to infer that both bacterial and spider SMaseDs originated from a common, broadly conserved domain family, the glycerophosphoryl diester phosphodiesterases. We also identify a unique sequence/structure motif present in both SMaseDs but not in the ancestral family, supporting SMaseD origin through a single divergence event in either bacteria or spiders, followed by lateral gene transfer from one lineage to the other.