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PubMed1980

A novel reaction catalyzed by unadenylylated glutamine synthetase from Escherichia coli. AMP-dependent synthesis of pyrophosphate and L-Glutamate from orthophosphate and L-glutamine.

The unadenylylated, manganese form of glutamine synthetase (L-glutamate: ammonia ligase (ADP forming), EC 6.3.1.2 from Escherichia coli catalyzes a novel, AMP-dependent (reversible) synthesis of pyrophosphate and L-glutamate from orthophosphate and L-glutamine: Formula (See Text). The hydrolysis of the L-glutamine amide bond is coupled to the stoichiometric synthesis of pyrophosphate, although as PPi accumulates, additional hydrolysis of L-glutamine occurs in a secondary reaction catalyzed by the [manganese x enzyme x AMP x PPi] complex. The synthesis of PPi probably occurs at the subunit catalytic site in the positions normally occupied by the beta, gamma-phosphates of ATP. To promote PPi synthesis, AMP apparently binds to the subunit catalytic site rather than to the allosteric inhibitor site; equilibrium binding results suggest that Pi directs the binding of AMP to the active site. In this reaction, Mg2+ will not substitute for Mn2+, and adenylylated glutamine synthetase is inactive. Pyrophosphate is synthesized by the unadenylylated, manganese enzyme at approximately 2% of the rate of that of ATP in the reverse biosynthetic reaction. If P1 is replaced by arsenate, the enzymatic rate of the AMP-supported hydrolysis of L-glutamine is 100-fold faster than is PPi synthesis and is one-half the rate of the ADP-supported, irreversible arsenolysis of L-glutamine. This latter activity also is supported by GMP and IMP, suggesting that the catalytic site of glutamine synthetase has a rather broad specificity for the nucleotide base. The reactions supported by AMP directly relate to the mechanism of glutamine synthetase catalysis.

Adenosine Monophosphate↗
PubMed1979

The effect of tRNA and tryptophanyl adenylate on limited proteolysis of beef pancreas tryptophanyl-tRNA synthetase.

Limited proteolysis of tryptophanyl-tRNA synthetase was used to detect changes in the enzyme molecule in the presence of substrates. Trypsinolysis of each of the two identical subunits occurs in succession from the N-terminus as follows: 60 leads to 51 leads to 40 leads to 24 kilodaltons. The transition 51 leads to 40 is hindered in tryptophanyl adenylate.enzyme complex. Yeast tRNATrp accelerates the first steps of hydrolysis and decelerates the transition 40 leads to 24. Once tRNATrp is added to the synthetase.adenylate complex, the protective effect of the adenylate disappears. The same effects are found also in the presence of tRNATrp oxidized with NaI04 and tRNATrp lacking the 3'-terminal adenosine. Oxidized tRNATrp (but not tRNATrp without the 3'-A) accelerates tryptophan-dependent hydrolysis of ATP catalyzed by the enzyme. A scheme is proposed for the interaction of yeast tRNATrp with beef pancreas tryptophanyl-tRNA synthetase involving the association of tRNA with a positively charged site(s) of the enzyme and the changes in the conformation of enzyme manifesting itself in unfolding of the acidic N-terminal fragment of the polypeptide chain and in the exposure of the adenylate.

Adenosine Monophosphate↗
PubMed2000

Kinetic mechanism of the beta-lactam synthetase of Streptomyces clavuligerus.

Streptomyces clavuligerus beta-lactam synthetase (beta-LS) was recently demonstrated to catalyze an early step in clavulanic acid biosynthesis, the ATP/Mg(2+)-dependent intramolecular closure of the beta-amino acid N(2)-(carboxyethyl)-L-arginine (CEA) to the monocyclic beta-lactam deoxyguanidinoproclavaminic acid (DGPC). Here we investigate the steady-state kinetic mechanism of the beta-LS-catalyzed reaction to better understand this unprecedented secondary metabolic enzyme. Initial velocity patterns were consistent with a sequential ordered bi-ter kinetic mechanism. Product inhibition studies with PP(i) and DGPC demonstrated competitive inhibition versus their cognate substrates ATP and CEA, respectively, and noncompetitive inhibition against their noncognate substrates. To clarify the order of substrate binding, the truncated substrate analogue N(2)-(carboxymethyl)-L-arginine was synthesized and demonstrated uncompetitive inhibition versus ATP and competitive patterns versus CEA. These data are consistent with ordered substrate binding, with ATP binding first, an abortive enzyme-DGPC complex, and PP(i) released as the last product. The pH dependence of V and V/K was determined and suggests that residues with a pK of 6.5 and 9.3 must be ionized for optimal activity. These observations were considered in the context of investigations of the homologous primary metabolic enzyme asparagine synthetase B, and a chemical mechanism is proposed that is consistent with the kinetic mechanism.

Adenosine Triphosphate↗
PubMed1980

Beta-selenaproline as competitive inhibitor of proline activation.

Beta-Selenaproline, a proline analog having the beta-methylene group substituted by a selenium atom, has been tested in ATP-PPi exchange reaction catalyzed by either Escherichia coli or rat liver aminoacyl-tRNA synthetases. It has been shown that with both enzymatic systems beta-selenaproline does not give rise to ATP-PPi exchange, but specifically inhibits proline activation. The inhibition is of fully competitive type and the Ki values, lower than the Km values for proline, show that beta-selenaproline binds to the synthetases with high affinity. The inability to form the complex with AMP, taking into account also the behavior of gamma-selenaproline and other proline analogs, has been ascribed to the presence of the selenium atom in the beta-position.

Adenosine Triphosphate↗
PubMed2000

Catalytic peptide of human glutaminyl-tRNA synthetase is essential for its assembly to the aminoacyl-tRNA synthetase complex.

Human glutaminyl-tRNA synthetase (QRS) is one of several mammalian aminoacyl-tRNA synthetases (ARSs) that form a macromolecular protein complex. To understand the mechanism of QRS targeting to the multi-ARS complex, we analyzed both exogenous and endogenous QRSs by immunoprecipitation after overexpression of various Myc-tagged QRS mutants in human embryonic kidney 293 cells. Whereas a deletion mutant containing only the catalytic domain (QRS-C) was targeted to the multi-ARS complex, a mutant QRS containing only the N-terminal appended domain (QRS-N) was not. Deletion mapping showed that the ATP-binding Rossman fold was necessary for targeting of QRS to the multi-ARS complex. Furthermore, exogenous Myc-tagged QRS-C was co-immunoprecipitated with endogenous QRS. Since glutaminylation of tRNA was dramatically increased in cells transfected with the full-length QRS, but not with either QRS-C or QRS-N, both the QRS catalytic domain and the N-terminal appended domain were required for full aminoacylation activity. When QRS-C was overexpressed, arginyl-tRNA synthetase and p43 were released from the multi-ARS complex along with endogenous QRS, suggesting that the N-terminal appendix of QRS is required to keep arginyl-tRNA synthetase and p43 within the complex. Thus, the eukaryote-specific N-terminal appendix of QRS appears to stabilize the association of other components in the multi-ARS complex, whereas the C-terminal catalytic domain is necessary for QRS association with the multi-ARS complex.

Adenosine Triphosphate↗
PubMed1981

Inhibition of cytochrome c oxidase function by dicyclohexylcarbodiimide.

Dicyclohexylcarbodiimide (DCCD) reacted with beef heart cytochrome c oxidase in inhibit the proton-pumping function of this enzyme and to a lesser extent to inhibit electron transfer. The modification of cytochrome c oxidase in detergent dispersion or in vesicular membranes was in subunits II-IV. Labelling followed by fragmentation studies showed that there is one major site of modification in subunit III. DCCD was also incorporated into several sites in subunit II and at least one site of subunit IV. The major site in subunit III has a specificity for DCCD at least one order of magnitude greater than that of other sites (in subunits II and IV). Its modification could account for all of the observed effects of the reagent, at least for low concentrations of DCCD. Labelling of subunit II by DCCD was blocked by prior covalent attachment of arylazidocytochrome c, a cytochrome c derivative which binds to the high-affinity binding site for the substrate. The major site of DCCD binding in subunit III was sequenced. The label was found in glutamic acid 90 which is in a sequence of eight amino acids remarkably similar to the DCCD-binding site within the proteolipid protein of the mitochondrial ATP synthetase.

Amino Acid Sequence↗
PubMed1984

Incorporation of 1,N6-ethenoadenosine into the 3' terminus of tRNA using T4 RNA ligase. 1. Preparation of yeast tRNAPhe derivatives.

The 3'-terminal A-C-C-A sequence of yeast tRNAPhe has been modified by replacing either adenosine 76 or 73 with the fluorescent analogues 1,N6-ethenoadenosine (epsilon A) or 2-aza-1,N6-ethenoadenosine (aza-epsilon A). T4 RNA ligase was used to join the nucleoside 3',5'-bisphosphates to the 3' end of the tRNA which was shortened by one [tRNAPhe(-A)] or four [tRNAPhe(-ACCA)] nucleotides. It was found that the base-paired 3'-terminal cytidine 72 in tRNAPhe(-ACCA) is a more efficient acceptor in the ligation reaction than the unpaired cytidine 75 at the A-C-C terminus of tRNAPhe(-A). This finding indicates that the mobility of the accepting nucleoside substantially influences the ligation reaction, the efficiency being higher the lower the mobility. This conclusion is corroborated by the observation that the ligation reaction with the double-stranded substrate exhibits a positive temperature dependence rather than a negative one as found for single-stranded acceptors. The replacement of the 3'-terminal adenosine 76 with epsilon A and aza-epsilon A leads to moderately fluorescent tRNAPhe derivatives, which are inactive in the aminoacylation reaction. A number of other tRNAs (Met, Ser, Glu, Lys and Leu-specific tRNAs both from yeast and Escherichia coli) are also inactivated by epsilon A incorporation. Replacement of adenosine 73 followed by repair of the C-C-A end using nucleotidyl transferase leads to tRNAPhe derivatives which are fully active in the aminoacylation reaction and in polyphenylalanine synthesis. The fluorescence of epsilon A and aza-epsilon A at position 73 is virtually completely quenched, suggesting a stacked arrangement of bases around this position. There is no fluorescence increase when the epsilon A-labeled tRNAPhe is complexed with phenylalanyl-tRNA synthetase, elongation factor Tu, or ribosomes. These observations indicate that the stacked conformation of the 3' terminus is not changed appreciably in these complexes.

Adenosine↗
PubMed1985

Kinetic mechanism of glutaminyl-tRNA synthetase from human placenta.

The order of interaction of substrates and products with human placental glutaminyl-tRNA synthetase was investigated in the aminoacylation reaction by using the steady-state kinetic methods. The initial velocity patterns obtained from both the glutamine-ATP and glutamine-tRNA substrate pairs were intersecting, whereas ATP and tRNA showed double competitive substrate inhibition. Dead-end inhibition studies with an ATP analog, tripolyphosphate, showed uncompetitive inhibition when tRNA was the variable substrate. The product inhibition studies revealed that PPi was an uncompetitive inhibitor with respect to tRNA. The noncompetitive inhibition by AMP versus tRNA was converted to uncompetitive by increasing the concentration of glutamine from 0.05 to 0.5 mM. These and other kinetic patterns obtained from the present study, together with our earlier finding that this human enzyme catalyzed the ATP-PPi exchange reaction in the absence of tRNA, enable us to propose a unique two-step, partially ordered sequential mechanism, with tRNA as the leading substrate, followed by random addition of ATP and glutamine. The products may be released in the following order: AMP, PPi and then glutaminyl-tRNA. The proposed mechanism involves both a quarternary complex including all three substrates and the intermediary formation of an enzyme-bound aminoacyl adenylate, common to the usual sequential and ping-pong mechanisms, respectively, for other aminoacyl-tRNA synthetases.

Amino Acyl-tRNA Synthetases↗
PubMed2001

L-pyroglutamic acid inhibits energy production and lipid synthesis in cerebral cortex of young rats in vitro.

In the present study we investigated the effects of L-pyroglutamic acid (PGA), which predominantly accumulates in the inherited metabolic diseases glutathione synthetase deficiency (GSD) and gamma-glutamylcysteine synthetase deficiency (GCSD), on some in vitro parameters of energy metabolism and lipid biosynthesis. We evaluated the rates of CO2 production and lipid synthesis from [U-14C]acetate, as well as ATP levels and the activities of creatine kinase and of the respiratory chain complexes I-IV in cerebral cortex of young rats in the presence of PGA at final concentrations ranging from 0.5 to 3 mM. PGA significantly reduced brain CO2 production by 50% at the concentrations of 0.5 to 3 mM, lipid biosynthesis by 20% at concentrations of 0.5 to 3 mM and ATP levels by 52% at the concentration of 3 mM. Regarding the enzyme activities, PGA significantly decreased NADH:cytochrome c oxireductase (complex I plus CoQ plus complex III) by 40% at concentrations of 0.5-3.0 mM and cytochrome c oxidase activity by 22-30% at the concentration of 3.0 mM, without affecting the activities of succinate dehydrogenase, succinate:DCPIP oxireductase (complex II), succinate:cytochrome c oxireductase (complex II plus CoQ plus complex III) or creatine kinase. The results strongly indicate that PGA impairs brain energy production. If these effects also occur in humans, it is possible that they may contribute to the neuropathology of patients affected by these diseases.

Animals↗
PubMed2006

Molecular dynamics simulations show that bound Mg2+ contributes to amino acid and aminoacyl adenylate binding specificity in aspartyl-tRNA synthetase through long range electrostatic interactions.

Molecular recognition between the aminoacyl-tRNA synthetase enzymes and their cognate amino acid ligands is essential for the faithful translation of the genetic code. In aspartyl-tRNA synthetase (AspRS), the co-substrate ATP binds preferentially with three associated Mg2+ cations in an unusual, bent geometry. The Mg2+ cations play a structural role and are thought to also participate catalytically in the enzyme reaction. Co-binding of the ATP x Mg3(2+) complex was shown recently to increase the Asp/Asn binding free energy difference, indicating that amino acid discrimination is substrate-assisted. Here, we used molecular dynamics free energy simulations and continuum electrostatic calculations to resolve two related questions. First, we showed that if one of the Mg2+ cations is removed, the Asp/Asn binding specificity is strongly reduced. Second, we computed the relative stabilities of the three-cation complex and the 2-cation complexes. We found that the 3-cation complex is overwhelmingly favored at ordinary magnesium concentrations, so that the protein is protected against the 2-cation state. In the homologous LysRS, the 3-cation complex was also strongly favored, but the third cation did not affect Lys binding. In tRNA-bound AspRS, the single remaining Mg2+ cation strongly favored the Asp-adenylate substrate relative to Asn-adenylate. Thus, in addition to their structural and catalytic roles, the Mg2+ cations contribute to specificity in AspRS through long range electrostatic interactions with the Asp side chain in both the pre- and post-adenylation states.

Adenosine Monophosphate↗
PubMed1997

HIV-1 p17 and IFN-gamma both induce fructose 1,6-bisphosphatase.

The p17 matrix protein of the human immunodeficiency virus type 1 (HIV-1) plays a crucial role in AIDS pathogenesis. It orchestrates viral assembly and directs the preintegration complex to the nucleus of infected cells. Recently, the three-dimensional structure of p17 was shown to resemble that of interferon-gamma (IFN-gamma), suggesting that both proteins might share analogous functions. We demonstrate that in monocytes, p17 shares with IFN-gamma the ability to induce 1alpha-hydroxylase activity and to activate fructose 1,6-bisphosphatase gene expression in the presence of 25-hydroxyvitamin D3. However, p17 does not bind to the IFN-gamma cell membrane receptor and fails to increase expression of IFN-gamma-induced proteins, such as tryptophanyl-tRNA synthetase, Fc gammaRI, and HLA DR or B7/BB1 antigens. Altogether, our results raise the possibility that the structural resemblance between p17 and IFN-gamma causes the selective activation of a common pathway resulting in the production of 1,25-dihydroxyvitamin D3. We also found that unlike IFN-gamma, p17 increases the intracellular ATP content. Since transport of the HIV-1 preintegration complex through the nuclear membrane is an ATP-dependent process, our observation suggests that p17 plays a double role in this active transport, not only by acting as a chaperone molecule but also by recruiting the necessary energy for this process.

25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗
PubMed1998

Snapshot of a phosphorylated substrate intermediate by kinetic crystallography.

The ATP-dependent enzyme dethiobiotin synthetase from Escherichia coli catalyses the formation of dethiobiotin from CO2 and 7, 8-diaminopelargonic acid. The reaction is initiated by the formation of a carbamate and proceeds through a phosphorylated intermediate, a mixed carbamic phosphoric anhydride. Here, we report the crystal structures at 1.9- and 1.6-A resolution, respectively, of the enzyme-MgATP-diaminopelargonic acid and enzyme-MgADP-carbamic-phosphoric acid anhydride complexes, observed by using kinetic crystallography. Reaction initiation by addition of either NaHCO3 or diaminopelargonic acid to crystals already containing cosubstrates resulted in the accumulation of the phosphorylated intermediate at the active site. The phosphoryl transfer step shows inversion of the configuration at the phosphorus atom, consistent with an in-line attack by the carbamate oxygen onto the phosphorus atom of ATP. A key feature in the structure of the complex of the enzyme with the reaction intermediate is two magnesium ions, bridging the phosphates at the cleavage site. These magnesium ions compensate the negative charges at both phosphate groups after phosphoryl transfer and contribute to the stabilization of the reaction intermediate.

Adenosine Triphosphate↗
PubMed1996

Unusual enzyme characteristics of aspartyl-tRNA synthetase from hyperthermophilic archaeon Pyrococcus sp. KOD1.

The aspA gene, encoding the aspartyl-tRNA synthetase (AspRS) from the hyperthermophilic archaeon Pyrococcus sp. KOD1, was expressed in Escherichia coli. The KOD1 AspRS, which was purified to homogeneity and was shown to be functional in dimeric form, aminoacylated tRNA from KOD1. The optimum temperature for this activity was 65 degrees C, which was lower than that for the cell growth of KOD1 (85 degrees C). However, it increased to 75 degrees C by the addition of polyamine molecules, such as putrescine, spermine, and spermidine. Analysis of the thermal denaturations of the enzyme and of KOD1-tRNA indicated that neither of them was denatured at temperatures below 70 degrees C. These results suggest polyamine is one of the factors which are required to stabilize the AspRS-tRNA complex in vivo. In order to determine whether the nucleotide triphosphate (NTP) is required for Asp-tRNA synthesis, the aminoacylation was examined in the presence of each of the four NTPs. AspRS most effectively aminoacylated tRNA in the presence of ATP. However, we also found that the enzyme aminoacylated it even in the presence of GTP and UTP as well. Archaeon synthetase may have an interesting system to utilize other NTPs than ATP. The extreme conditions of early life may have given rise to these unique characteristics which then disappeared from developed organisms through evolution.

Archaea↗
PubMed1989

Formylglycinamide ribonucleotide synthetase from Escherichia coli: cloning, sequencing, overproduction, isolation, and characterization.

The purL gene of Escherichia coli encoding the enzyme formylglycinamidine ribonucleotide (FGAM) synthetase which catalyzes the conversion of formylglycinamide ribonucleotide (FGAR), glutamine, and MgATP to FGAM, glutamate, ADP, and Pi has been cloned and sequenced. The mature protein, as deduced by the structural gene sequence, contains 1628 amino acids and has a calculated Mr of 141,418. Comparison of the purL control region to other pur loci control regions reveals a common region of dyad symmetry which may be the binding site for the "putative" repressor protein. Construction of an overproducing strain permitted purification of the protein to homogeneity. N-Terminal sequence analysis and comparison of glutamine binding domain sequences (Ebbole & Zalkin, 1987) confirm the amino acid sequence deduced from the gene sequence. The purified protein exhibits glutaminase activity of 0.02% the normal turnover, and NH3 can replace glutamine as a nitrogen donor with a Km = 1 M and a turnover of 3 min-1 (2% glutamine turnover). The enzyme forms an isolable (1:1) complex with glutamine: t1/2 is 22 min at 4 degrees C. This isolated complex is not chemically competent to complete turnover when FGAR and ATP are added, demonstrating that ammonia and glutamine are not covalently bound as a thiohemiaminal available to complete the chemical conversion to FGAM. hydroxylamine trapping experiments indicate that glutamine is bound covalently to the enzyme as a thiol ester. Initial velocity and dead-end inhibition kinetic studies on FGAM synthetase are most consistent with a sequential mechanism in which glutamine binds followed by rapid equilibrium binding of MgATP and then FGAR. Incubation of [18O]FGAR with enzyme, ATP, and glutamine results in quantitative transfer of the 18O to Pi.

Amino Acid Sequence↗
PubMed1991

Apparent ATP-linked succinate thiokinase activity and its relation to nucleoside diphosphate kinase in mitochondrial matrix preparations from rabbit.

The relative abilities of ATP and GTP to support succinyl-CoA synthesis by mitochondrial matrix fractions prepared from rabbit heart and liver mitoplasts were investigated. The activity supported by ATP in rabbit heart preparations was less than 15% of that obtained with GTP, while no ATP-supported activity was observed in rabbit liver preparations. However, the addition of 30 micromolar GDP to matrix fractions from either heart or liver stimulated the ATP-supported activity to 40% of that observed with GTP, and the further addition of bovine liver nucleoside diphosphate kinase in the presence of 8 microM added GDP increased the activity to near that observed with GTP. The specific activity of nucleoside diphosphate kinase assayed directly in mitochondrial matrix from heart was about 15% of the specific activity of ATP-supported succinate thiokinase induced upon adding GDP. Evidence for a complex between nucleoside diphosphate kinase and succinate thiokinase in mitochondrial matrix from rabbit heart was obtained by glycerol density gradient centrifugation. It is proposed that binding of nucleoside diphosphate kinase to succinate thiokinase activates the former enzyme, accounts for the ATP-supported succinyl-CoA synthetase activity observed, and is involved in the channeling of high energy phosphate from GTP produced in the Krebs cycle to the ATP pool.

Adenosine Triphosphate↗
PubMed1989

Escherichia coli carbamoyl-phosphate synthetase: domains of glutaminase and synthetase subunit interaction.

Three catalytic domains of the Escherichia coli carbamoyl-phosphate synthetase (EC 6.3.5.5) have been identified in previous studies. These include the glutamine amide-N transfer domain in the carboxyl-terminal half of the glutaminase component and at least two adenine nucleotide binding sites in the synthetase component. To delineate the domains involved in subunit interactions, we have examined the effects of deletions and point mutations in the glutaminase and synthetase subunits on formation of the alpha beta holoenzyme. Deletion of the amino-terminal third of the glutaminase subunit abolishes interactions with the synthetase subunit, suggesting that this domain functions to stabilize the complex. Two subunit binding domains have been identified in the synthetase subunit. They are homologous to one another and are located in the amino-terminal and central regions of the synthetase component. These domains are adjacent to regions of the synthetase previously proposed to be involved in ATP binding and, possibly, activation of CO2. The new data enlarge the definition of the structural and functional domains in the two interdependent components of carbamoyl-phosphate synthetase.

Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzin↗
PubMed1978

The mechanism of the aminoacylation of transfer ribonucleic acid. The kinetics and stoichiometry of the lysis of aminoacyl-tRNA.

It is often stated that the aminoacylation of transfer RNA proceeds in discrete steps: (formula: see article). If this is a complete description of the reaction, the reverse overall formation of ATP should not be more rapid than the formation of Enz . (AA approximately AMP). We show for four different amino acid:tRNA ligases that lysis of AA-tRNA (with PPi and AMP) to ATP is faster than lysis of AA-tRNA (with AMP only) to Enz . (AA approximately AMP). This requires that the transition state proceeds from a quaternary complex of PPi, AMP, AA-tRNA and Enz. From the law of microscopic reversibility, this requires that in the forward reaction the AA-tRNA bond be formed before PPi leaves the enzyme complex. Therefore, the forward reaction passes through the quaternary complex Enz . ATP . AA . tRNA. (In view of recent evidence of the specific requirement of two cations, the complex is accurately described as senary).

Adenosine Monophosphate↗
PubMed1988

Molecular cloning and nucleotide sequence of the nuclear PET122 gene required for expression of the mitochondrial COX3 gene in S. cerevisiae.

The nuclear PET122 gene from S. cerevisiae is necessary for translation of a single mitochondrial mRNA that encodes subunit III of cytochrome c oxidase. We report here the cloning and nucleotide sequence of PET122, and properties of the predicted protein product, which consists of 242 residues. Analysis of PET122-lacZ translational fusions confirms that the PET122 coding region is translated in vivo and indicates that the PET122 protein product is targeted to mitochondria. A 117 residue domain located in the carboxy-terminal half of the PET122 protein, at least part of which is shown by characterization of mutants to be critical for PET122 function, exhibits 24% identity and 59% similarity to a portion of the catalytic domain of E. coli alanyl-tRNA synthetase. However, pet122 mutants are not defective in mitochondrial translation per se, as would be expected if PET122 encoded a tRNA synthetase. Instead, the PET122 protein may carry out one or more activities in common with tRNA synthetases, such as binding of ATP or RNA.

Amino Acid Sequence↗