Evolutionary relationship between translation initiation factor eIF-2 gamma and selenocysteine-specific elongation factor SELB: Change of function intranslation factors

Eubacterial and eukaryotic translation initiation systems have very little in common, and therefore the evolutionary events that gave rise to these tw o disparate systems are difficult to ascertain. One common feature is the p resence of initiation, elongation, and release factors belonging to a large GTPase superfamily. One of these initiation factors, the gamma subunit of initiation factor 2 (eIF-2 gamma), is found only in eukaryotes and archaeba cteria. We have sequenced eIF-2 gamma gene fragments from representative di plomonads, parabasalia, and microsporidia and used these new sequences toge ther with new archaebacterial homologues to examine the phylogenetic positi on of eIF-2 gamma within the GTPase superfamily. The archaebacterial and eu karyotic eIF-2 gamma proteins are found to be very closely related, and are in turn related to SELB, the selenocysteine-specific elongation factor fro m eubacteria. The overall topology of the GTPase tree further suggests that the eIF-2 gamma/SELB group may represent an ancient subfamily of GTPases t hat diverged prior to the last common ancestor of extant life.