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

Abstract

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.

A Multifunctional Repeated Motif Is Present in Human Bifunctional tRNA Synthetase

“…Overexpression of these motifs in transgenic flies resulted in a decrease of fertility (17). This result further supports the physiological importance of these motifs.…”

“…Although the EPRS repeats were not essential for the activity of the connected tRNA synthetase (8), overexpression of these repeats in Drosophila resulted in sterility (17). Similarly, a mutation of the gene for yeast Arc1p induced slow growth and cold sensitivity of the cell, although it was not essential for the activities of the bound tRNA synthetases (41).…”

“…1A). Secondary structure predictions of the motif suggested that it would form an ␣-helix (6,17,18). To determine the structure and stability in folded configuration, one of the repeated motifs (EPRS-R1) was subjected to NMR and DSC analyses.…”

“…This result further supports the physiological importance of these motifs. It has been proposed that they may serve as a template for assembly of multisynthetase complex or as an anchor to link the complex to other cellular components, such as protein synthesis machinery (6,17).…”

Caenorhabditis elegans Evolves a New Architecture for the Multi-aminoacyl-tRNA Synthetase Complex

“…The lack of in vivo experimental approaches acutely hindered our understanding of the underlying mechanisms. The only experimental approach was previously reported in Drosophila (30). It was shown that overexpression of the repeated motifs that link GluRS and ProRS in Drosophila leads to a sterility of the transgenic flies.…”