EVIDENCE FOR THE EARLY DIVERGENCE OF TRYPTOPHANYL-TRANSFER-RNA AND TYROSYL-TRANSFER-RNA SYNTHETASES

Each amino acid is attached to its cognate tRNA by a distinct aminoacy l-tRNA synthetase (aaRS). The conventional evolutionary view is that t he modern complement of synthetases existed prior to the divergence of eubacteria and eukaryotes. Thus comparisons of prokaryotic and eukary otic aminoacyl-tRNA synthetases of the same type (charging specificity ) should show greater sequence similarities than comparisons between s ynthetases of different types-and this is almost always so, However, a recent study [Ribas de Pouplana L, Furgier M, Quinn CL, Schimmel P (1 996) Proc Natl Acad Sci USA 93:166-170] suggested that tryptophanyl(Tr pRS) and tyrosyl-tRNA (TyrRS) synthetases of the Eucarya (eukaryotes) are more similar to each other than either is to counterparts in the B acteria (eubacteria). Here, we reexamine the evolutionary relationship s of TyrRS and TrpRS using a broader range of taxa, including new sequ ence data from the Archaea (archaebacteria) as well as species of Euca rya and Bacteria. Our results differ from those of Ribas de Pouplana e t al.: All phylogenetic methods support the separate monophyly of TrpR S and TyrRS. We attribute this result to the inclusion of the archaeal data which might serve to reduce long branch effects possibly associa ted with eukaryotic TrpRS and TyrRS sequences. Furthermore, reciprocal ly rooted phylogenies of TrpRS and TyrRS sequences confirm the closer evolutionary relationship of Archaea to eukaryotes by placing the root of the universal tree in the Bacteria.