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Schimmel P. Alanine transfer RNA synthetase: structure-function relationships and molecular recognition of transfer RNA. ADVANCES IN ENZYMOLOGY AND RELATED AREAS OF MOLECULAR BIOLOGY 2006; 63:233-70. [PMID: 2407064 DOI: 10.1002/9780470123096.ch4] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- P Schimmel
- Department of Biology, Massachusetts Institute of Technology, Cambridge 02139
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2
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Houman F, Rho SB, Zhang J, Shen X, Wang CC, Schimmel P, Martinis SA. A prokaryote and human tRNA synthetase provide an essential RNA splicing function in yeast mitochondria. Proc Natl Acad Sci U S A 2000; 97:13743-8. [PMID: 11087829 PMCID: PMC17646 DOI: 10.1073/pnas.240465597] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Mitochondrial leucyl-tRNA synthetase (LeuRS) in the yeast Saccharomyces cerevisiae provides two essential functions. In addition to aminoacylation, LeuRS functions in RNA splicing. The details of how it came to act in splicing are not known. Here we show that Mycobacterium tuberculosis and human mitochondrial LeuRSs can substitute in splicing for the S. cerevisiae mitochondrial LeuRS. Mutations of yeast mitochondrial LeuRS that had previously been shown to abolish splicing activity also eliminate splicing by the M. tuberculosis enzyme. These results suggest the role of LeuRS in splicing in yeast mitochondria results from features of the enzyme that are broadly conserved in evolution. These features are not likely to be designed for splicing per se, but instead have been adopted in yeast for that purpose.
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Affiliation(s)
- F Houman
- Department of Biology and Biochemistry, University of Houston, 3201 Cullen, Houston, TX 77204-5513, USA
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3
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Landrieu I, Vandenbol M, Härtlein M, Portetelle D. Mitochondrial asparaginyl-tRNA synthetase is encoded by the yeast nuclear gene YCR24c. EUROPEAN JOURNAL OF BIOCHEMISTRY 1997; 243:268-73. [PMID: 9030748 DOI: 10.1111/j.1432-1033.1997.0268a.x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
One of the open reading frames located on yeast Saccharomyces cerevisiae chromosome III, YCR24c, appeared to code for a protein of unknown function, but the predicted sequence showed similarity with asparaginyl-tRNA synthetase from Escherichia coli, with 38% amino acid identity. There is a putative mitochondrial targeting signal at the N-terminus of the YCR24c product. Northern blot analysis of total RNA from a wild-type strain sigma1278b confirmed that YCR24c was transcribed. Disruption of the chromosomal copy of YCR24c in a respiratory-competent haploid cell induced a petite phenotype, but did not affect cell viability. This respiratory-defective phenotype is typical for a mutation in a nuclear gene that induces a non-functional mitochondrial protein synthesis system. The protein encoded by YCR24c was expressed in Escherichia coli in a histidine-tagged form and isolated. The enzyme aminoacylated unfractionated Escherichia coli tRNA with asparagine. These results identified YCR24c as the structural gene for yeast mitochondrial asparaginyl-tRNA synthetase.
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Affiliation(s)
- I Landrieu
- Unité de Microbiologie, Faculté Universitaire des Sciences Agronomiques de Gembloux, Belgium
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4
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Landès C, Perona JJ, Brunie S, Rould MA, Zelwer C, Steitz TA, Risler JL. A structure-based multiple sequence alignment of all class I aminoacyl-tRNA synthetases. Biochimie 1995; 77:194-203. [PMID: 7647112 DOI: 10.1016/0300-9084(96)88125-9] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The superimposable dinucleotide fold domains of MetRS, GlnRS and TyrRS define structurally equivalent amino acids which have been used to constrain the sequence alignments of the 10 class I aminoacyl-tRNA synthetases (aaRS). The conservation of those residues which have been shown to be critical in some aaRS enables to predict their location and function in the other synthetases, particularly: i) a conserved negatively-charged residue which binds the alpha-amino group of the amino acid substrate; ii) conserved residues within the inserted domain bridging the two halves of the dinucleotide-binding fold; and iii) conserved residues in the second half of the fold which bind the amino acid and ATP substrate. The alignments also indicate that the class I synthetases may be partitioned into two subgroups: a) MetRS, IleRS, LeuRS, ValRS, CysRS and ArgRS; b) GlnRS, GluRS, TyrRS and TrpRS.
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Affiliation(s)
- C Landès
- Centre de Génétique Moléculaire, Université P & M Curie, Gif-sur-Yvette, France
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5
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Chemical modification and mutagenesis studies on zinc binding of aminoacyl-tRNA synthetases. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)82266-9] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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6
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Nada S, Chang P, Dignam J. Primary structure of the gene for glycyl-tRNA synthetase from Bombyx mori. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53008-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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7
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Shiba K, Schimmel P. Tripartite functional assembly of a large class I aminoacyl tRNA synthetase. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(18)50003-x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
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8
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Hohmann S, Thevelein JM. The cell division cycle gene CDC60 encodes cytosolic leucyl-tRNA synthetase in Saccharomyces cerevisiae. Gene X 1992; 120:43-9. [PMID: 1398122 DOI: 10.1016/0378-1119(92)90007-c] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
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.
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Affiliation(s)
- S Hohmann
- Laboratorium voor Moleculaire Cellbiologie, Katholieke Universiteit te Leuven, Flanders, Belgium
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9
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Schimmel P, Shepard A, Shiba K. Intron locations and functional deletions in relation to the design and evolution of a subgroup of class I tRNA synthetases. Protein Sci 1992; 1:1387-91. [PMID: 1303756 PMCID: PMC2142098 DOI: 10.1002/pro.5560011018] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Affiliation(s)
- P Schimmel
- Department of Biology, Massachusetts Institute of Technology, Cambridge 02139
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10
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Abstract
Signature sequences are contiguous patterns of amino acids 10-50 residues long that are associated with a particular structure or function in proteins. These may be of three types (by our nomenclature): superfamily signatures, remnant homologies, and motifs. We have performed a systematic search through a database of protein sequences to automatically and preferentially find remnant homologies and motifs. This was accomplished in three steps: 1. We generated a nonredundant sequence database. 2. We used BLAST3 (Altschul and Lipman, Proc. Natl. Acad. Sci. U.S.A. 87:5509-5513, 1990) to generate local pairwise and triplet sequence alignments for every protein in the database vs. every other. 3. We selected "interesting" alignments and grouped them into clusters. We find that most of the clusters contain segments from proteins which share a common structure or function. Many of them correspond to signatures previously noted in the literature. We discuss three previously recognized motifs in detail (FAD/NAD-binding, ATP/GTP-binding, and cytochrome b5-like domains) to demonstrate how the alignments generated by our procedure are consistent with previous work and make structural and functional sense. We also discuss two signatures (for N-acetyltransferases and glycerol-phosphate binding) which to our knowledge have not been previously recognized.
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Affiliation(s)
- R P Sheridan
- Medical Research Division, Lederle Laboratories, American Cyanamid Corp., Pearl River, New York 10965
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11
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Isoleucyl-tRNA synthetase from the ciliated protozoan Tetrahymena thermophila. DNA sequence, gene regulation, and leucine zipper motifs. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(18)42874-8] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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12
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Ghosh G, Pelka H, Schulman LH, Brunie S. Activation of methionine by Escherichia coli methionyl-tRNA synthetase. Biochemistry 1991; 30:9569-75. [PMID: 1911742 DOI: 10.1021/bi00104a002] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
In the present work, we have examined the function of three amino acid residues in the active site of Escherichia coli methionyl-tRNA synthetase (MetRS) in substrate binding and catalysis using site-directed mutagenesis. Conversion of Asp52 to Ala resulted in a 10,000-fold decrease in the rate of ATP-PPi exchange catalyzed by MetRS with little or no effect on the Km's for methionine or ATP or on the Km for the cognate tRNA in the aminoacylation reaction. Substitution of the side chain of Arg233 with that of Gln resulted in a 25-fold increase in the Km for methionine and a 2000-fold decrease in kcat for ATP-PPi exchange, with no change in the Km for ATP or tRNA. These results indicate that Asp52 and Arg233 play important roles in stabilization of the transition state for methionyl adenylate formation, possibly directly interacting with complementary charged groups (ammonium and carboxyl) on the bound amino acid. Primary sequence comparisons of class I aminoacyl-tRNA synthetases show that all but one member of this group of enzymes has an aspartic acid residue at the site corresponding to Asp52 in MetRS. The synthetases most closely related to MetRS (including those specific for Ile, Leu, and Val) also have a conserved arginine residue at the position corresponding to Arg233, suggesting that these conserved amino acids may play analogous roles in the activation reaction catalyzed by each of these enzymes. Trp305 is located in a pocket deep within the active site of MetRS that has been postulated to form the binding cleft for the methionine side chain.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- G Ghosh
- Department of Developmental Biology and Cancer, Albert Einstein College of Medicine, Bronx, New York 10461
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13
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Miller WT, Hill KA, Schimmel P. Evidence for a "cysteine-histidine box" metal-binding site in an Escherichia coli aminoacyl-tRNA synthetase. Biochemistry 1991; 30:6970-6. [PMID: 1712632 DOI: 10.1021/bi00242a023] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Escherichia coli alanyl-tRNA synthetase contains the sequence Cys-X2-Cys-X6-His-X2-His. This motif is distinct from the zinc fingers of DNA-binding proteins but has some similarity to the Cys-X2-Cys-X4-His-X4-Cys zinc-binding motif of retroviral gag proteins, where it has a role in RNA packaging. In Ala-tRNA synthetase, this sequence is located in an amino-terminal domain which has the site for docking the acceptor end of the tRNA near the bound aminoacyl adenylate and is immediately adjacent in the sequence to the location of a mutation that affects the specificity of tRNA recognition. We show here that Ala-tRNA synthetase contains approximately 1 mol of zinc/mol of polypeptide and that addition of the zinc chelator 1,10-phenanthroline inhibits its aminoacylation activity. Conservative mutations of specific cysteine or histidine residues in the "Cys-His box" destabilize and inactivate the enzyme, whereas mutations of intervening amino acids do not inactivate. The possibility that this motif can bind zinc (or cobalt) was demonstrated with a synthetic 22 amino acid peptide that is based on the sequence of the alanine enzyme. The peptide-cobalt complex has the spectral characteristics of tetrahedral coordination geometry. The results establish that the Cys-His box motif of Ala-tRNA synthetase has the potential to form a specific complex with zinc (at least in the context of a synthetic peptide analogue) and suggest that this motif is important for enzyme stability/activity.
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Affiliation(s)
- W T Miller
- Department of Biology, Massachusetts Institute of Technology, Cambridge 02139
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14
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Nucleotide and deduced amino acid sequence of human threonyl-tRNA synthetase reveals extensive homology to the Escherichia coli and yeast enzymes. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)92906-6] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
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15
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Abstract
A yeast nuclear gene, designated MSK1, has been selected from a yeast genomic library by transformation of a respiratory deficient mutant impaired in acylation of mitochondrial lysine tRNA. This gene confers a respiratory competent phenotype and restores the mutant's ability to acylate the mitochondrial lysine tRNA. The amino acid sequence of the protein encoded by MSK1 is homologous to yeast cytoplasmic lysyl-tRNA synthetase and to the product of the herC gene, which has recently been suggested to code for the Escherichia coli enzyme. These observations indicate that MSK1 codes for the lysyl-tRNA synthetase of yeast mitochondria. Several regions of high primary sequence conservation have been identified in the bacterial and yeast lysyl-tRNA synthetases. These domains are also present in the aspartyl- and asparaginyl-tRNA synthetases, further confirming the notion that all three present-day enzymes originated from a common ancestral gene. The most conserved domain, located near the carboxyl terminal ends of this group of synthetases is characterized by a cluster of glycines and is also highly homologous to the carboxyl-terminal region of the E. coli ammonia-dependent asparagine synthetase. A catalytic function of the carboxyl terminal domain is indicated by in vitro mutagenesis of the yeast mitochondrial lysyl-tRNA synthetase. Replacement of any one of three glycine residues by alanine and in one case by aspartic acid completely suppresses the activity of the enzymes, as evidenced by the inability of the mutant genes to complement an msk1 mutant, even when present in high copy. Other mutations result in partial loss of activity. Only one glycine replacement affects the stability of the protein in vivo. The observed presence of a homologous domain in asparagine synthetase, which, like the aminoacyl-tRNA synthetases, catalyzes the formation of an aminoacyladenylate, suggests that the glycine-rich sequence is part of a catalytic site involved in binding of ATP and of the aminoacyladenylate intermediate.
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Affiliation(s)
- D L Gatti
- Department of Biological Sciences, Columbia University, New York, NY 10027
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16
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Horner J, Champney WS, Samuels R. Characteristics of a leucine aminoacyl transfer RNA synthetase from Tritrichomonas augusta. Int J Parasitol 1991; 21:275-7. [PMID: 1869366 DOI: 10.1016/0020-7519(91)90023-z] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
This study has investigated the characteristics of a leucine aminoacyl transfer RNA synthetase enzyme from Tritrichomonas augusta. Differential centrifugation and DEAE-cellulose column chromatography were used for partial enzyme purification. The column purification increased the synthetase activity 125-fold over the unfractionated cell extract. The conditions for maximum [3H] leucine charging were 37 degrees C for 20 min, with protein at 180 micrograms ml-1 using yeast leucine tRNA as an acceptor. The optimal reaction conditions were 14 mM-Mg acetate, 3 mM-ATP, 3 mM-spermidine and 5.5 mM-putrescine. Acceptor activity with T. augusta transfer RNA was 8-fold higher than with yeast transfer RNA and 25-fold higher than with Escherichia coli transfer RNA. The partially purified enzyme fraction had comparable changing activities for both leucine and valine.
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Affiliation(s)
- J Horner
- Department of Biochemistry, University of Illinois, Urbana 61801
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17
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Zagorski W, Castaing B, Herbert CJ, Labouesse M, Martin R, Slonimski PP. Purification and characterization of the Saccharomyces cerevisiae mitochondrial leucyl-tRNA synthetase. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)52278-x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
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18
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Mirande M. Aminoacyl-tRNA synthetase family from prokaryotes and eukaryotes: structural domains and their implications. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1991; 40:95-142. [PMID: 2031086 DOI: 10.1016/s0079-6603(08)60840-5] [Citation(s) in RCA: 200] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Affiliation(s)
- M Mirande
- Laboratoire d'Enzymologie, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France
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19
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Glutamyl-tRNA synthetases of Bacillus subtilis 168T and of Bacillus stearothermophilus. Cloning and sequencing of the gltX genes and comparison with other aminoacyl-tRNA synthetases. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(17)44745-4] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
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20
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Abstract
We describe a collection of nuclear respiratory-defective mutants (pet mutants) of Saccharomyces cerevisiae consisting of 215 complementation groups. This set of mutants probably represents a substantial fraction of the total genetic information of the nucleus required for the maintenance of functional mitochondria in S. cerevisiae. The biochemical lesions of mutants in approximately 50 complementation groups have been related to single enzymes or biosynthetic pathways, and the corresponding wild-type genes have been cloned and their structures have been determined. The genes defined by an additional 20 complementation groups were identified by allelism tests with mutants characterized in other laboratories. Mutants representative of the remaining complementation groups have been assigned to one of the following five phenotypic classes: (i) deficiency in cytochrome oxidase, (ii) deficiency in coenzyme QH2-cytochrome c reductase, (iii) deficiency in mitochondrial ATPase, (iv) absence of mitochondrial protein synthesis, and (v) normal composition of respiratory-chain complexes and of oligomycin-sensitive ATPase. In addition to the genes identified through biochemical and genetic analyses of the pet mutants, we have cataloged PET genes not matched to complementation groups in the mutant collection and other genes whose products function in the mitochondria but are not necessary for respiration. Together, this information provides an up-to-date list of the known genes coding for mitochondrial constituents and for proteins whose expression is vital for the respiratory competence of S. cerevisiae.
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Affiliation(s)
- A Tzagoloff
- Department of Biological Sciences, Columbia University, New York, New York 10027
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21
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Mitochondrial Aminoacyl-?RNA Synthetases. ACTA ACUST UNITED AC 1990. [PMID: 2247606 DOI: 10.1016/s0079-6603(08)60625-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
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22
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23
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Toth MJ, Schimmel P. Deletions in the large (beta) subunit of a hetero-oligomeric aminoacyl-tRNA synthetase. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(19)40149-x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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24
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Martindale DW, Gu ZM, Csank C. Isolation and complete sequence of the yeast isoleucyl-tRNA synthetase gene (ILS1). Curr Genet 1989; 15:99-106. [PMID: 2663194 DOI: 10.1007/bf00435455] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The isoleucyl-tRNA synthetase gene (ILS1) from the yeast Saccharomyces cerevisiae was cloned and sequenced. This gene was initially cloned because it cross-hybridizated to what is now presumed to be the isoleucyl-tRNA synthetase gene (cupC) from the protozoan Tetrahymena thermophila. The ILS1 gene was determined to be 1,072 amino acids in length. A comparison with a recently published sequence of ILS1 from another laboratory (Englisch et al. 1987) was made and differences noted. Two promoter elements were detected, one for general amino acid control and one for constitutive transcription. A heat shock protein (hsp70) gene (probably SSA3) was found 237 bp upstream from the ILS1 translation start site. The ILS1 amino acid sequence was compared to isoleucyl-tRNA synthetases from other organisms, as well as to valyl-, leucyl- and methionyl-tRNA synthetases. Regions of conservation between these enzymes were found.
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Affiliation(s)
- D W Martindale
- Department of Microbiology, Macdonald College of McGill University, Quebec, Canada
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25
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26
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Schwob E, Sanni A, Fasiolo F, Martin RP. Purification of the yeast mitochondrial methionyl-tRNA synthetase. Common and distinctive features of the cytoplasmic and mitochondrial isoenzymes. EUROPEAN JOURNAL OF BIOCHEMISTRY 1988; 178:235-42. [PMID: 3060359 DOI: 10.1111/j.1432-1033.1988.tb14448.x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Yeast-mitochondrial methionyl-tRNA synthetase was purified 1060-fold from mitochondrial matrix proteins of Saccharomyces cerevisiae using a four-step procedure based on affinity chromatography (heparin-Ultrogel, tRNA(Met)-Sepharose, Agarose-hexyl-AMP) to yield to a single polypeptide of high specific activity (1800 U/mg). Like the cytoplasmic methionyl-tRNA synthetase (Mr 85,000), the mitochondrial isoenzyme is a monomer, but of significantly smaller polypeptide size (Mr 65,000). In contrast, the corresponding enzyme of Escherichia coli is a dimer (Mr 152,000) made up of identical subunits. The measured affinity constants of the purified mitochondrial enzyme for methionine and tRNA(Met) are similar to those of the cytoplasmic isoenzyme. However, the two yeast enzymes exhibit clearly different patterns of aminoacylation of heterologous yeast and E. coli tRNA(Met). Furthermore, polyclonal antibodies raised against the two proteins did not show any cross-reactivity by inhibition of enzymatic activity and by the highly sensitive immunoblotting technique, indicating that the two enzymes share little, if any, common antigenic determinants. Taken together, our results further support the belief that the yeast mitochondrial and cytoplasmic methionyl-tRNA synthetases are different proteins coded for by two distinct nuclear genes. Like the yeast cytoplasmic aminoacyl-tRNA synthetases, the mitochondrial enzymes displayed affinity for immobilized heparin. This distinguishes them from the corresponding enzymes of E. coli. Such an unexpected property of the mitochondrial enzymes suggests that they have acquired during evolution a domain for binding to negatively charged cellular components.
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Affiliation(s)
- E Schwob
- Institut de Biologie Moléculaire et Cellulaire du CNRS, Laboratoire de Biochemie, Strasbourg, France
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