Archaeabacterial seryl-tRNA synthetases: adaptation to extreme environments and evolutionary analysis.

The aminoacyl-tRNA synthetases are ubiquitous enzymes which catalyze a crucial step of the cell life, the specific attachment of amino acids to their cognate tRNA. The amino acid sequences of three archaeal seryl-tRNA synthetases (SerRS) from Haloarcula marismortui and Methanococcus jannaschii, both belonging to the group of Euryarchaeota, and from Sulfolobus solfataricus, of the group of Crenarchaeota, were aligned with other eubacterial and eukaryal available SerRS sequences. In an attempt to identify some features of adaptation to extreme environments of these organisms, amino acid composition and amino acid substitutions between mesophilic and thermophilic SerRS were analyzed. In addition, universal phylogenetic trees of SerRS including the three known archaeal sequences, rooted by the threonyl-tRNA synthetases were inferred. Amino acid analyses of the SerRS revealed two ways of adaptation to thermophilic environments between the Eubacteria and the Archaea; most of the usually described amino acid substitutions were nonsignificant in the case of archaeal thermophilic SerRS and most amino acid composition biases seemed to be linked to the genome G+C content pressure. The phylogenetic analysis of the SerRS showed the Archaea to be paraphyletic, H. marismortui emerging with the Gram-positive Bacteria, M. jannaschii being near the root of the tree, and S. solfataricus branching with Eucarya.

Seryl-tRNA synthetase from the extreme halophile Haloarcula marismortui--isolation, characterization and sequencing of the gene and its expression in Escherichia coli.

The seryl-tRNA synthetase from the extreme halophilic archaebacterium Haloarcula marismortui, belonging to the group Euryarchaeota, has been purified and its hyperhalophilic behavior demonstrated by activity and stability tests in KCl, NaCl and MgCl2 solutions. Although the natural external environment of this archaebacterium is rich in sodium ions and poor in potassium ions, the converse being the case in the bacterial cytosol. there is no large significant difference in activity and stability in vitro of the enzyme between solutions of NaCl and KCl. Low, but not high, concentrations of MgCl2 stabilize the enzyme. The enzyme aminoacylates tRNA from Escherichia coli even under the high salt conditions of the assay. A fluorescence study indicated that low salt denaturation of the hyperhalophilic enzyme is a biphasic process. The hyperhalophilic enzyme demonstrated immunological reactivity with antisera against the catalytic domain of the homologous E. coli enzyme. The gene coding for the H. marismortui enzyme has been isolated and sequenced. The derived amino acid sequence is the first of a hyperhalophilic aminoacyl-tRNA synthetase. The wild-type gene and a mutant gene with a deletion of the halophile-specific insertion were expressed in E. coli using the T7 RNA polymerase and the Thiofusion expression systems. None of the expressed proteins were enzymically active. A structural model has been produced by comparison with other seryl-tRNA synthetases which illustrates the high negative-charge density of the surface of the hyperhalophilic enzyme.