A gene fusion event in the evolution of aminoacyl-tRNA synthetases

The genes of glutamyl- and prolyl-tRNA synthetases (GluRS and ProRS) are or ganized differently in the three kingdoms of the tree of life. In bacteria and archaea, distinct genes encode the two proteins. In several organisms f rom the eukaryotic phylum of coelomate metazoans, the two polypeptides are carried by a single polypeptide chain to form a bifunctional protein, The l inker region is made of imperfectly repeated units also recovered as singul ar or plural elements connected as N-terminal or C-terminal polypeptide ext ensions in various eukaryotic aminoacyl-tRNA synthetases, Phylogenetic anal ysis points to the monophyletic origin of this polypeptide motif appended t o six different members of the synthetase family, belonging to either of th e two classes of aminoacyl-tRNA synthetases, In particular, the monospecifi c GluRS and ProRS from Caenorhabditis elegans, an acoelomate metazoan, exhi bit this recurrent motif as a C-terminal or N-terminal appendage, respectiv ely. Our analysis of the extant motifs suggests a possible series of events responsible for a gene fusion that gave rise to the bifunctional glutamyl- prolyl-tRNA synthetase through recombination between genomic sequences enco ding the repeated units. (C) 2000 Federation of European Biochemical Societ ies.