ON THE ORIGIN OF PROTEIN-SYNTHESIS FACTORS - A GENE DUPLICATION FUSION MODEL/

Sequence similarity has given rise to the proposal that IF-2, EF-G, an d EF-Tu are related through a common ancestor. We evaluate this propos ition and whether the relationship can be extended to other factors of protein synthesis. Analysis of amino acid sequence similarity gives s tatistical support for an evolutionary affiliation among IF-1, IF-2, I F-3, EF-Tu, EF-Ts, and EF-G and suggests further that this association is a result of gene duplication/fusion events. In support of this mec hanism, the three-dimensional structures of IF-3, EF-Tu, and EF-G disp lay a predictable domain structure and overall conformational similari ty. The model that we propose consists of three consecutives duplicati on/fusion events which would have taken place before the divergence of the three superkingdoms: eubacteria, archaea, and eukaryotes. The roo t of this protein superfamily tree would be an ancestor of the modern IF-1 gene sequence. The repeated fundamental motif of this protein sup erfamily is a small RNA binding domain composed of two alpha-helices p acked along side of an antiparallel beta-sheet.