Exons encoding the highly conserved part of human glutaminyl-tRNA synthetase

Evolution of the aminoacyl-tRNA synthetase family and the organization of the Drosophila glutamyl-prolyl-tRNA synthetase gene. Intron/exon structure of the gene, control of expression of the two mRNAs, selective advantage of the multienzyme complex

In Drosophila, glutamyl-prolyl-tRNA synthetase is a multifunctional synthetase encoded by a unique gene and composed of three domains: the amino- and carboxy-terminal domains catalyze the aminoacylation of glutamic acid and proline tRNA species, respectively, and the central domain is made of 75 amino acids repeated six times amongst which 46 are highly conserved and constitute the repeated motifs [Cerini, C., Kerjan, P., Astier, M., Gratecos, D., Mirande, M. & Sémériva, M. (1991) EMBO J. 10, 4267-4277]. The intron/exon organization of the Drosophila gene reveals the presence of six exons among which four are in the 5'-end encoding glutamic acid activity. Only one exon encodes the repeated motifs. A comparison of introns positions, intron classes and intron/exon boundaries in the Drosophila gene and in its human counterpart is compatible with the intron-early hypothesis presiding, at least in part, to the evolution of the synthetases. The full-length fly protein is encoded by a 6.1-kb mRNA which is expressed throughout development. In addition, a shorter transcript encompasses the 3'-end of the cDNA and it is especially abundant in 5-10-h embryos until the first larval stage. Expression of these two mRNAs seems to be controlled by two independent promoters. The 6.1-kb mRNA promoter is probably localized in the 5'-end of the gene. The small mRNA promoter resides in the 4th intron and evidence is provided that the mRNA encodes only the domain corresponding to prolyl-tRNA synthetase and is functional in vivo. Finally, transgenic flies have been established by using constructs corresponding to the three domains of the protein. Overexpression of the repeated motifs leads to a sterility of the flies that suggests a role of these motifs in linking the multienzyme complex to an, as yet, unknown structure of the protein synthesis apparatus.

The aminoacyl-tRNA synthetase family: modules at work

The combined use of molecular and structural biology techniques has proved very efficient in elucidating structure-function relationships in aminoacyl-tRNA synthetases. Our present understanding of this family of enzymes is based on two main unifying principles: (i) division into two different classes, corresponding to two different modes of ATP binding and attachment of the activated amino acid to the last nucleotide of tRNA (either 2'OH or 3'OH of the ribose) by two different catalytic mechanisms and two structural domains with completely different folding, and (ii) the modular organization into separate and additional domains that we are just beginning to understand. Sequence analysis complements very nicely existing structural, biochemical and genetic results and makes them more general, leading to verifiable predictions.

Origin of glutaminyl-tRNA synthetase: an example of palimpsest?

Sequence data and evolutionary arguments suggest that a similarity may exist between the C-terminal end of glutaminyl-tRNA synthetase (GlnRS) and the catalytic domain of glutamine amidotransferases (GATs). If true, this would seem to imply that the amidation reaction of the Glut-tRNA(Gln) complex was the evolutionary precursor of the direct tRNA(Gln) aminoacylation pathway. Since the C-terminal end of GlnRS does not now have an important functional role, it can be concluded that this sequence contains vestiges that lead us to believe that it represents a palimpsest. This sequence still conserves the remains of the evolutionary transition: amidation reaction-->aminoacylation reaction. This may be important in deciding which mechanism gave origin to the genetic code organization. These observations, together with results obtained by Gatti and Tzagoloff [J. Mol. Biol. (1991) 218:557-568], lead to the hypothesis that the class I aminoacyl-tRNA synthetases (ARSs) may be homologous to the GATs of the trpG subfamily, while the class II ARSs may be homologous to the GATs of the purF subfamily. Overall, this seems to point to the existence of an intimate evolutionary link between the proteins involved in the primitive metabolism and aminoacyl-tRNA synthetases.

The human gene encoding tryptophanyl-tRNA synthetase: interferon-response elements and exon-intron organization

Recently, we cloned and sequenced the cDNA encoding human tryptophanyl-tRNA synthetase (hWRS) [Frolova et al., Gene 109 (1991) 291-296]. Independently, it has been shown that this protein is induced by interferons (IFN) gamma and alpha [Fleckner et al., Proc. Natl. Acad. Sci. USA 88 (1991) 11520-11524; Rubin et al., J. Biol. Chem. 266 (1991) 24245-24248]. This unusual feature of a housekeeping enzyme raises the problem of how the gene is regulated. Since at present the genomic structure of hWRS is unknown, this issue remains unsolved. Here, the exon-intron organization of hWRS has been deciphered. This gene consists of at least 12 exons that span more than 35 kb of DNA. At least two alternative noncoding exons precede ten coding exons. Upstream from the first exon, two GGAAAN(N/-)GAAA sequences, which are considered to be IFN-stimulating response elements (ISRE), have been revealed. The same consensus was also found in the intron region in close vicinity to the 5' end of the second exon. Thus, the IFN-stimulated synthesis of hWRS is presumably due to gene activation at the transcriptional level. Alignment of hWRS amino acid sequences has shown that exons V to XI of hWRS encode regions of structural similarity with bacterial WRS, whereas the N-terminal portion of the protein encoded by exons II to IV exhibits no homology with bacterial WRS.(ABSTRACT TRUNCATED AT 250 WORDS)

Mammalian tryptophanyl-tRNA synthetases

Aminoacyl-tRNA synthetases of higher organisms are far less studied compared to their prokaryotic and unicellular eukaryotic counterparts. However, many aminoacyl-tRNA synthetases from multi-cellular organisms exhibit certain features not yet described for the same enzymes of bacteria or yeast. Tryptophanyl-tRNA synthetases (TrpRS) are among the most thoroughly studied mammalian enzymes of this group. TrpRS are Zn(2+)-dependent, dimeric, class I aminoacyl-tRNA synthetases with known amino acid sequence for four different mammalian orders. TrpRS is not associated in a stable multi-synthetase complex, although it exhibits a long N-terminal extension absent from bacterial TrpRS. The human gene encoding TrpRS belongs to the interferon-responsive gene family and TrpRS activity drastically increases after interferon gamma induction. For unknown reasons TrpRS is overproduced in pancreas of Ruminantia. Other data on TrpRS available so far are summarized and briefly discussed here.