tRNA recognition and evolution of determinants in seryl-tRNA synthesis

We have analyzed the evolution of recognition of tRNAs(Ser) by seryl-tRNA s ynthetases, and compared it to other type 2 tRNAs, which contain a long ext ra arm. In Eubacteria and chloroplasts this type of tRNA is restricted to t hree families: tRNA(Leu), tRNA(Ser) and tRNA(Tyr). tRNA(Leu) and tRNA(Ser) also carry a long extra arm in Archaea, Eukarya and all organelles with the exception of animal mitochondria. In contrast, the long extra arm of tRNA( Tyr) is far less conserved: it was drastically shortened after the separati on of Archaea and Eukarya from Eubacteria, and it is also truncated in anim al mitochondria. The high degree of phylogenetic divergence in the length o f tRNA variable arms, which are recognized by both class I and class II ami noacyl-tRNA synthetases, makes type 2 tRNA recognition an ideal system with which to study how tRNA discrimination may have evolved in tandem with the evolution of other components of the translation machinery.