Genomics and the evolution of aminoacyl-tRNA synthesis.

Translation is the process by which ribosomes direct protein synthesis using the genetic information contained in messenger RNA (mRNA). Transfer RNAs (tRNAs) are charged with an amino acid and brought to the ribosome, where they are paired with the corresponding trinucleotide codon in mRNA. The amino acid is attached to the nascent polypeptide and the ribosome moves on to the next codon. Thus, the sequential pairing of codons in mRNA with tRNA anticodons determines the order of amino acids in a protein. It is therefore imperative for accurate translation that tRNAs are only coupled to amino acids corresponding to the RNA anticodon. This is mostly, but not exclusively, achieved by the direct attachment of the appropriate amino acid to the 3'-end of the corresponding tRNA by the aminoacyl-tRNA synthetases. To ensure the accurate translation of genetic information, the aminoacyl-tRNA synthetases must display an extremely high level of substrate specificity. Despite this highly conserved function, recent studies arising from the analysis of whole genomes have shown a significant degree of evolutionary diversity in aminoacyl-tRNA synthesis. For example, non-canonical routes have been identified for the synthesis of Asn-tRNA, Cys-tRNA, Gln-tRNA and Lys-tRNA. Characterization of non-canonical aminoacyl-tRNA synthesis has revealed an unexpected level of evolutionary divergence and has also provided new insights into the possible precursors of contemporary aminoacyl-tRNA synthetases.

Methanococcus jannaschii prolyl-cysteinyl-tRNA synthetase possesses overlapping amino acid binding sites.

The protein translation apparatus of Methanococcus jannaschii possesses the unusual enzyme prolyl-cysteinyl-tRNA synthetase (ProCysRS), a single enzyme that attaches two different amino acids, proline and cysteine, to their cognate tRNA species. Measurement of the ATP-PP(i) exchange reaction revealed that amino acid activation, the first reaction step, differs for the two amino acids. While Pro-AMP can be formed in the absence of tRNA, Cys-AMP synthesis is tRNA-dependent. Studies with purified tRNAs indicate that tRNA(Cys) promotes cysteine activation. The k(cat) values of wild-type ProCysRS for tRNA prolylation (0.09 s(-1)) and cysteinylation (0.02 s(-1)) demonstrate that both aminoacyl-tRNAs are synthesized with comparable rates, the cysteinyl-tRNA synthetase activity being only 4.5-fold lower than prolyl-tRNA synthetase activity. Kinetic analysis of ProCysRS mutant enzymes, generated by site-directed mutagenesis, shows glutamate at position 103 to be critical for proline binding, and proline at position 100 to be involved in cysteine binding. The proximity in ProCysRS of amino acid residues affecting binding of either cysteine or proline strongly suggests that structural elements of the two amino acid binding sites overlap.

Regulation of aldose reductase expression in rat astrocytes in culture.

Aldose reductase (AR) is known to be responsible for many side effects of diabetes. In the present work, we studied the effects of various extracellular signals on the regulation of the expression of AR in astrocytes in culture, by determining its enzymatic activity or its mRNA level. We found that basic fibroblast growth factor (bFGF), acidic fibroblast growth factor (aFGF), epidermal growth factor (EGF), and hypertonic NaCl were able to increase the expression of AR in astrocytes. A superinduction was found when bFGF was combined with hypertonicity. We also observed that AR activity was independent of glucose concentration in the culture medium. However, when the concentration of glucose in the culture medium was under 1 g/l, bFGF did not increase the activity of AR. Thus, when glucose is depleted, the regulation of AR expression by bFGF does not operate. In addition, AR does not seem to be involved in control of astrocyte proliferation, in contrast to the effects reported on other cell types. These results indicate that AR is expressed in astrocytes and that its expression is upregulated by hypertonicity but also by FGFs and EGF. This suggests that in these cells, AR elicits some regulatory functions.