Phylogenomic analysis of the α proteasome gene family from early-diverging eukaryotes

We employed a phylogenomic approach to study the evolution of or subunits of the proteasome gene family from early diverging eukaryotes. BLAST similarity searches of the Giardia lamblia genome identified all seven or proteasome genes characteristic of eukaryotes from the crown group. In addition, a PCR strategy for the amplification of multiple or subunit sequences generated single or proteasome products for representatives of the Kinetoplastida (Leishmania major), the Parabasalia (Trichomonas vaginalis), and the Microsporidia (Vairimorpha sp., Nosema sp., Endoreticulata sp., and Spraguea lophii). The kinetoplastid Trypanosoma cruzi and the eukaryote crown group Acanthamoeba castellanii yielded two distinct or proteasome genes each. The presence of seven distinct or proteasome genes in G. lamblia, one of the earliest-diverging eukaryotes, indicates that the or proteasome gene family evolved rapidly from a minimum of one gene in Archaea to seven or more in Eukarya. Results from the phylogenomic analysis are consistent with the idea that the Diplomonida (as represented by G. lamblia), the Kinetoplastida, the Parabasalia, and the Microsporidia diverged after the duplication events that originated the or proteasome gene family. A model for the early origin and evolution of the proteasome gene family is presented.