The same Arabidopsis gene encodes both cytosolic and mitochondrial alanyl-tRNA synthetases.

Abstract

In plants, all aminoacyl-tRNA synthetases are nuclearly encoded, despite the fact that their activities are required in the three protein-synthesizing cell compartments (cytosol, mitochondria, and chloroplasts). To investigate targeting of these enzymes, we cloned cDNAs encoding alanyl-tRNA synthetase (AlaRS) and the corresponding nuclear gene, A LATS, from Arabidopsis by using degenerate polymerase chain reaction primem based on highly conserved regions shared between known AlaRSs from other organisms. Analysis of the transcription of the gene showed the presence of two potential translation initiation codons in some ALATS mRNAs. Translation from the upstream AUG would generate an N-terminal extension with features characteristic of mitochondrial targeting peptides. A polyclonal antibody raised against part of the Arabidopsis AlaRS revealed that the Arabidopsis cytosolic and mitochondrial AlaRSs are immunologically similar, suggesting that both isoforms are encodsd by the ALATS gene. In vitro experiments confirmed that two polypeptides can be translated from ALATS transcripts, with most ribosomes initiating on the downstream AUG t o give the shorter polypeptide corresponding in size to the cytosolic enzyme. The ability of the presequence encoded between the two initiation codons to direct polypeptides to mitochondria was demonstrated by expression of fusion proteins in tobacco protoplasts and i n yeast. We conclude that the ALATS gene encodes both the cytosolic and the mitochondrial forms of AlaRS, depending on which of the two AUG codons is used to initiate translation.

Duplication and Quadruplication of Arabidopsis thaliana Cysteinyl- and Asparaginyl-tRNA Synthetase Genes of Organellar Origin

“…However, as in other eukaryotes, the situation is more complicated, as there are several cases where one gene has replaced the function of another because its gene product is dual-targeted to two compartments. Dual-targeting to the cytosol and mitochondria was shown for Arabidopsis alanyl-tRNA synthetase (Mireau et al 1996) and is probably also the case for several other plant aaRSs (Small et al 1999). Dualtargeting is also possible to mitochondria and plastids, as demonstrated for Arabidopsis histidyl-tRNA ) and methionyl-tRNA (Menand et al 1998) synthetases.…”

Nuclear Photosynthetic Gene Expression Is Synergistically Modulated by Rates of Protein Synthesis in Chloroplasts and Mitochondria

“…In addition to the prors1-3 and -4 mutations described in this article, knockout mutants for other mitochondrial aaRSs have been described; however, these were only impaired in embryo development. In the acd mutant, which shows reduced expression of the ALARS gene coding for an alanyl-tRNA synthetase targeted to both cytosol and mitochondria (Mireau et al, 1996), abortion of embryos at the globular stage is observed (Ge et al, 1998). In the edd1 mutant, inactivation of the glycyl-tRNA synthetase GLYRS caused the arrest of embryo development between the globular and heart stages.…”

A gene encoding 1‐aminocyclopropane‐1‐carboxylate (ACC) synthase produces two transcripts: elucidation of a conserved response

“…In plants, the alternative promoters that have been studied are primarily involved in N-terminal extensions utilizing alternative AUG initiation sites. They regulate subcellular targeting (Luo et al, 1997;Mireau et al, 1996) or tissue-specific expression (Lumbreras et al, 1995), and their protein products retain similar enzymatic functions. To our knowledge, the only example in plants in which usage of an alternative promoter has been shown to result in a functionally unique product is a rice homeobox gene (Tamaoki et al, 1995).…”