Nuclear gene for mitochondrial leucyl-tRNA synthetase of Neurospora crassa: isolation, sequence, chromosomal mapping, and evidence that the leu-5 locus specifies structural information

The processing of human mitochondrial leucyl-tRNA synthetase in the insect cells

A His-tagged full-length cDNA of human mitochondrial leucyl-tRNA synthetase was expressed in a baculovirus system. The N-terminal sequence of the enzyme isolated from the mitochondria of insect cells was found to be IYSATGKWTKEYTL, indicating that the mitochondrial targeting signal peptide was cleaved between Ser39 and Ile40 after the enzyme precursor was translocated into mitochondria. The enzyme purified from mitochondria catalyzed the leucylation of Escherichia coli tRNA(1)(Leu)(CAG) and Aquifex aeolicus tRNA(Leu)(GAG) with higher catalytic activity in the leucylation of E. coli tRNA(Leu) than that previously expressed in E. coli without the N-terminal 21 residues.

Expression and characterization of the human mitochondrial leucyl-tRNA synthetase

A cDNA clone encoding the human mitochondrial leucyl-tRNA synthetase (mtLeuRS) has been identified from the EST databases. Analysis of the protein encoded by this cDNA indicates that the protein is 903 amino acids in length and contains a mitochondrial signal sequence that is predicted to encompass the first 21 amino acids. Sequence analysis shows that this protein contains the characteristic motifs of class I aminoacyl-tRNA synthetases and regions of high homology to other mitochondrial and bacterial LeuRS proteins. The mature form of this protein has been cloned and expressed in Escherichia coli. Gel filtration indicates that human mtLeuRS is active in a monomeric state, with an apparent molecular mass of 101 kDa. The human mtLeuRS is capable of aminoacylating E. coli tRNA(Leu). Its activity is inhibited at high levels of either monovalent or divalent cations. K(M) and k(cat) values for ATP:PP(i) exchange and for the aminoacylation reaction have been determined.

Sequence of the met-10+ locus of Neurospora crassa: homology to a sequence of unknown function in Saccharomyces cerevisiae chromosome 8

We have determined the sequence of the Neurospora crassa met-10+ gene and its flanking regions, and have isolated and analyzed cDNA clones for this region. We have identified two closely linked genes transcribed in the same orientation. The met-10+ gene is the downstream gene; an open reading frame (ORF) derived from five exons encodes a 475-amino-acid protein. The deduced protein lacks similarity to other characterized proteins. However, it exhibits a strong similarity to the product of an ORF of unknown function on Saccharomyces cerevisiae chromosome 8. This sequence similarity suggests functional equivalence and should facilitate identification of the function of met-10+ using gene disruptions in S. cerevisiae.

The cell division cycle gene CDC60 encodes cytosolic leucyl-tRNA synthetase in Saccharomyces cerevisiae

The cdc60 mutation (for cell division cycle) of the yeast, Saccharomyces cerevisiae, confers arrest at the START point of the cell cycle upon shift to the restrictive temperature [Bedard et al., Curr. Genet. 4 (1981) 205-214]. We have cloned the CDC60 gene by complementation of the temperature-sensitive phenotype. Sequence analysis revealed a single open reading frame of 3270 bp and the deduced amino acid sequence showed 50.5% sequence identity to the cytosolic leucyl-tRNA synthetase (LeuRS) from Neurospora crassa, implying that CDC60 encodes the corresponding yeast protein. Thus, CDC60 does not appear to be involved directly in the regulation of the cell cycle. Rather, the cdc60 mutation leads to cell-cycle arrest at the nutrient control point START due to a deficiency of charged leucyl-tRNA. The CDC60 gene product also shows homology to LeuRSs from other organisms and to aminoacyl-RS for isoleucine, valine and methionine.

Impaired affinity for phenylalanine in Escherichia coli phenylalanyl-tRNA synthetase mutant caused by Gly-to-Asp exchange in motif 2 of class II tRNA synthetases

Phenylalanyl-tRNA synthetase (PheRS; alpha 2 beta 2 subunit structure) is a member of class II of tRNA synthetases. We report here the genetic analysis of an Escherichia coli mutant strain which is auxotrophic for phenylalanine because it has a PheRS with a decreased affinity for phenylalanine. The mutant pheS gene encoding the PheRS alpha subunit was cloned and sequenced, and the deviation from the wild-type gene was found to result in a Gly191-to-Asp191 exchange. This alteration is located within motif 2, one of 3 conserved sequence motifs characteristic for class II aminoacyl-tRNA synthetases. Motif 2 may thus participate in the formation of the phenylalanine binding site in PheRS.