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

Eubacterial and eukaryotic translation initiation systems have very little in common, and therefore the evolutionary events that gave rise to these two disparate systems are difficult to ascertain. One common feature is the presence 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 archaebacteria. We have sequenced eIF-2 gamma gene fragments from representative diplomonads, parabasalia, and microsporidia and used these new sequences together with new archaebacterial homologues to examine the phylogenetic position of eIF-2 gamma within the GTPase superfamily. The archaebacterial and eukaryotic eIF-2 gamma proteins are found to be very closely related, and are in turn related to SELB, the selenocysteine-specific elongation factor from eubacteria. The overall topology of the GTPase tree further suggests that the eIF-2 gamma/SELB group may represent an ancient subfamily of GTPases that diverged prior to the last common ancestor of extant life.