The CsdA cysteine desulphurase promotes Fe/S biogenesis by recruiting Suf components and participates to a new sulphur transfer pathway by recruiting CsdL (ex‐YgdL), a ubiquitin‐modifying‐like protein

Cysteine desulphurases are primary sources of sulphur that can eventually be used for Fe/S biogenesis or thiolation of various cofactors and tRNA. Escherichia coli contains three such enzymes, IscS, SufS and CsdA. The importance of IscS and SufS in Fe/S biogenesis is well established. The physiological role of CsdA in contrast remains uncertain. We provide here additional evidences for a functional redundancy between the three cysteine desulphurases in vivo. In particular, we show that a deficiency in isoprenoid biosynthesis is the unique cause of the lethality of the iscS sufS mutant. Moreover, we show that CsdA is engaged in two separate sulphur transfer pathways. In one pathway, CsdA interacts functionally with SufE–SufBCD proteins to assist Fe/S biogenesis. In another pathway, CsdA interacts with CsdE and a newly discovered protein, which we called CsdL, resembling E1‐like proteins found in ubiquitin‐like modification systems. We propose this new pathway to allow synthesis of an as yet to be discovered thiolated compound.     

Structural Changes during Cysteine Desulfurase CsdA and Sulfur Acceptor CsdE Interactions Provide Insight into the trans-Persulfuration

In Escherichia coli, three cysteine desulfurases (IscS, SufS, and CsdA) initiate the delivery of sulfur for various biological processes such as the biogenesis of Fe-S clusters. The sulfur generated as persulfide on a cysteine residue of cysteine desulfurases is further transferred to Fe-S scaffolds (e.g. IscU) or to intermediate cysteine-containing sulfur acceptors (e.g. TusA, SufE, and CsdE) prior to its utilization. Here, we report the structures of CsdA and the CsdA-CsdE complex, which provide insight into the sulfur transfer mediated by the trans-persulfuration reaction. Analysis of the structures indicates that the conformational flexibility of the active cysteine loop in CsdE is essential for accepting the persulfide from the cysteine of CsdA. Additionally, CsdA and CsdE invoke a different binding mode than those of previously reported cysteine desulfurase (IscS) and sulfur acceptors (TusA and IscU). Moreover, the conservation of interaction-mediating residues between CsdA/SufS and CsdE/SufE further suggests that the SufS-SufE interface likely resembles that of CsdA and CsdE.     

In vitro biosynthesis of a universal t6A tRNA modification in Archaea and Eukarya

N⁶-threonylcarbamoyladenosine (t⁶A) is a modified nucleotide found in all transfer RNAs (tRNAs) decoding codons starting with adenosine. Its role is to facilitate codon–anticodon pairing and to prevent frameshifting during protein synthesis. Genetic studies demonstrated that two universal proteins, Kae1/YgjD and Sua5/YrdC, are necessary for t⁶A synthesis in Saccharomyces cerevisiae and Escherichia coli. In Archaea and Eukarya, Kae1 is part of a conserved protein complex named kinase, endopeptidase and other proteins of small size (KEOPS), together with three proteins that have no bacterial homologues. Here, we reconstituted for the first time an in vitro system for t⁶A modification in Archaea and Eukarya, using purified KEOPS and Sua5. We demonstrated binding of tRNAs to archaeal KEOPS and detected two distinct adenosine triphosphate (ATP)-dependent steps occurring in the course of the synthesis. Our data, together with recent reconstitution of an in vitro bacterial system, indicated that t⁶A cannot be catalysed by Sua5/YrdC and Kae1/YgjD alone but requires accessory proteins that are not universal. Remarkably, we observed interdomain complementation when bacterial, archaeal and eukaryotic proteins were combined in vitro, suggesting a conserved catalytic mechanism for the biosynthesis of t⁶A in nature. These findings shed light on the reaction mechanism of t⁶A synthesis and evolution of molecular systems that promote translation fidelity in present-day cells.     

The universal YrdC/Sua5 family is required for the formation of threonylcarbamoyladenosine in tRNA

Threonylcarbamoyladenosine (t⁶A) is a universal modification found at position 37 of ANN decoding tRNAs, which imparts a unique structure to the anticodon loop enhancing its binding to ribosomes in vitro. Using a combination of bioinformatic, genetic, structural and biochemical approaches, the universal protein family YrdC/Sua5 (COG0009) was shown to be involved in the biosynthesis of this hypermodified base. Contradictory reports on the essentiality of both the yrdC wild-type gene of Escherichia coli and the SUA5 wild-type gene of Saccharomyces cerevisiae led us to reconstruct null alleles for both genes and prove that yrdC is essential in E. coli, whereas SUA5 is dispensable in yeast but results in severe growth phenotypes. Structural and biochemical analyses revealed that the E. coli YrdC protein binds ATP and preferentially binds RNA^Thr lacking only the t⁶A modification. This work lays the foundation for elucidating the function of a protein family found in every sequenced genome to date and understanding the role of t⁶A in vivo.     

Commonality and diversity in tRNA substrate recognition in t6A biogenesis by eukaryotic KEOPSs

N ⁶-Threonylcarbamoyladenosine (t⁶A) is a universal and pivotal tRNA modification. KEOPS in eukaryotes participates in its biogenesis, whose mutations are connected with Galloway-Mowat syndrome. However, the tRNA substrate selection mechanism by KEOPS and t⁶A modification function in mammalian cells remain unclear. Here, we confirmed that all ANN-decoding human cytoplasmic tRNAs harbor a t⁶A moiety. Using t⁶A modification systems from various eukaryotes, we proposed the possible coevolution of position 33 of initiator tRNA^Met and modification enzymes. The role of the universal CCA end in t⁶A biogenesis varied among species. However, all KEOPSs critically depended on C32 and two base pairs in the D-stem. Knockdown of the catalytic subunit OSGEP in HEK293T cells had no effect on the steady-state abundance of cytoplasmic tRNAs but selectively inhibited tRNA^Ile aminoacylation. Combined with in vitro aminoacylation assays, we revealed that t⁶A functions as a tRNA^Ile isoacceptor-specific positive determinant for human cytoplasmic isoleucyl-tRNA synthetase (IARS1). t⁶A deficiency had divergent effects on decoding efficiency at ANN codons and promoted +1 frameshifting. Altogether, our results shed light on the tRNA recognition mechanism, revealing both commonality and diversity in substrate recognition by eukaryotic KEOPSs, and elucidated the critical role of t⁶A in tRNA^Ile aminoacylation and codon decoding in human cells.     

The nature of unstable insertion mutations and reversions in the locus cut of Drosophila melanogaster: molecular mechanism of transposition memory

The segment of the locus cut containing the mobile genetic element mdg4 (gypsy) insertions which induce unstable ct^MR2 and ct^MRpN10 mutations has been cloned. Both mutations depend on the insertion of mdg4 into the same sequence, which coincides with that in ct⁶ allele. The ct^MRpN10 mutation differs from ct^MR2 by additional insertion of a novel mobile element jockey into mdg4. Jockey is 2.8 kb long, represented by ˜2–100 copies per genome, very homogeneous and lacks long terminal repeats (LTRs). The excision of mdg4 takes place in stable ct⁺ reversions. On the other hand, a complete single LTR is retained in the case of unstable ct reversions characterized by a high level of reverse directed transpositions of mdg4 into the locus cut. The LTR serves as a guide for reinsertion of mdg4 itself or mdg4 with jockey into the same site of the genome. A possible mechanism of transposition memory (homologous recombination with extrachromosomal circular DNA) is discussed.     

Cross Kingdom Functional Conservation of the Core Universally Conserved Threonylcarbamoyladenosine tRNA Synthesis Enzymes

Threonylcarbamoyladenosine (t⁶A) is a universal modification located in the anticodon stem-loop of tRNAs. In yeast, both cytoplasmic and mitochondrial tRNAs are modified. The cytoplasmic t⁶A synthesis pathway was elucidated and requires Sua5p, Kae1p, and four other KEOPS complex proteins. Recent in vitro work suggested that the mitochondrial t⁶A machinery of Saccharomyces cerevisiae is composed of only two proteins, Sua5p and Qri7p, a member of the Kae1p/TsaD family (L. C. K. Wan et al., Nucleic Acids Res. 41:6332–6346, 2013, http://dx.doi.org/10.1093/nar/gkt322). Sua5p catalyzes the first step leading to the threonyl-carbamoyl-AMP intermediate (TC-AMP), while Qri7 transfers the threonyl-carbamoyl moiety from TC-AMP to tRNA to form t⁶A. Qri7p localizes to the mitochondria, but Sua5p was reported to be cytoplasmic. We show that Sua5p is targeted to both the cytoplasm and the mitochondria through the use of alternative start sites. The import of Sua5p into the mitochondria is required for this organelle to be functional, since the TC-AMP intermediate produced by Sua5p in the cytoplasm is not transported into the mitochondria in sufficient amounts. This minimal t⁶A pathway was characterized in vitro and, for the first time, in vivo by heterologous complementation studies in Escherichia coli. The data revealed a potential for TC-AMP channeling in the t⁶A pathway, as the coexpression of Qri7p and Sua5p is required to complement the essentiality of the E. coli tsaD mutant. Our results firmly established that Qri7p and Sua5p constitute the mitochondrial pathway for the biosynthesis of t⁶A and bring additional advancement in our understanding of the reaction mechanism.     

Analysis of the heteromeric CsdA-CsdE cysteine desulfurase, assisting Fe-S cluster biogenesis in Escherichia coli

Biogenesis of iron-sulfur (Fe-S) cluster-containing proteins relies on assistance of complex machineries. To date three systems, NIF, ISC, and SUF, were reported to allow maturation of Fe-S proteins. Here we report that the csdA-csdE (formally ygdK) genes of Escherichia coli constitute a sulfur-generating system referred to as CSD which also contributes to Fe-S biogenesis in vivo. This conclusion was reached by applying a thorough combination of both in vivo and in vitro strategies and techniques. Yeast two-hybrid analysis allowed us to show that CsdA and CsdE interact. Enzymology analysis showed that CsdA cysteine desulfurase activity is increased 2-fold in the presence of CsdE. Mass spectrometry analysis and site-directed mutagenesis showed that residue Cys-61 from CsdE acted as an acceptor site for sulfur provided by cysteine desulfurase activity of CsdA. Genetic investigations revealed that the csdA-csdE genes could act as multicopy suppressors of iscS mutation. Moreover, both in vitro and in vivo investigations pointed to a specific connection between the CSD system and quinolinate synthetase NadA.     

Stability studies on the newly discovered cyclic form of tRNA N6-threonylcarbamoyladenosine (ct6A)

A cyclic form of N⁶-threonylcarbamoyladenosine bearing an oxazolone moiety (ct⁶A) was discovered very recently at the position 37 in several tRNA sequences. Our study on the synthesized 5′,3′,2′-O-acetylated derivative of ct⁶A confirmed high stability of the modified nucleoside under physiological conditions (PBS buffer, pH 7.4) and revealed remarkable stability of the oxazolone ring in acidic (100 mM HCl, pH 1) and basic (0.1 mM NaOH, pH 10) conditions. This feature may allow for the post-synthetic conversion of t⁶A into ct⁶A in assembled oligoribonucleotides.     

Characterization of the tRNA and aminoacyl-tRNA synthetases of healthy and tumorous callus tissues from Nicotiana tabacum

Aminoacyl-tRNA synthetases extracted from healthy and crown gall tumor tissues (induced by Agrobacterium tumefaciens strain B6) from Nicotiana tabacum (strain Wisconsin 38) grown in vitro, showed the same ability to charge Phaseolus vulgaristRNA, for all the 15 amino acids tested. For each amino acid, optimal charging conditions (enzyme concentration, Mg²⁺/ATP ratios, K⁺ ion effects) have been determined with Phaseolus vulgaristRNA and were found to be the same whether aminoacyl-tRNA synthetases from healthy or tumor tissues were used. In each case, valyl- and glutamyl-tRNA synthetases were very sensitive to an excess of Mg²⁺ and K⁺ ions. Although tRNA's extracted from healthy and tumor tissues gave the same electrophoretic patterns, charging levels obtained with turner tRNAs were generally 45% higher than those obtained with tRNA's from healthy tissues.     

Mobilization of the transposable element mdg4 by hybrid dysgenesis generates a family of unstable cut mutations in Drosophila melanogaster

Summary A family of unstable mutations at the cut locus in Drosophila melanogaster was obtained under the conditions of hybrid dysgenesis (Gerasimova 1981, 1982). The in situ hybridization experiments have shown that, in the original unstable ct ^MR2 mutation, the 7B region of the X chromosome (where cut is located) contains a mobile dispersed genetic element, mdg4. All other unstable ct mutations derived from ct ^MR2 including visible and lethal alleles and unstable ct ⁺ reversions, also contain mdg4 in the 7B region. The X chromosomes of the parent strain (wild type) do not contain mdg4 at all. All stable revertants derived from ct ^MR2, from other unstable ct mutations, or from ct lethals lost mdg4 from the 7B region. The ct ^MR2 X chromosome does not contain P-elements, although a few copies are present in the autosomes. The instability of the ct ^MR2./ct ^MR2 strain remained at a high level for 50 generations (1.5 years) and then rapidly decreased. A new cross with an MRh12/Cy strain (originally used for dysgenesis induction and containing a number of P-elements) increased the instability to a level exceeding the original one. The data strongly suggest that unstable ct mutations in our system are induced by transpositions of mdg4, possibly activated by P-elements.     

Ultraviolet light-induced responses of an mfd mutant of escherichia coli B/r having a slow rate of dimer excision

A mutant of Eschirichia coli B/r designated mfd has drastically reduced ability to exhibit “mutation frequency decline” (MFD) the irreversible loss of potential suppressor mutations which occurs when protein synthesis is briefly inhibited after irradiation with U.V. We have found that the initial rate of thymine dimer excision in the mfd mutant is only about one-third that of its mfd⁺ parent strain after a UV dose of 400 erg/mm². The yield of UV-induced Tyr⁺ revertants is 4–10 times higher in the mfd strain than in the mfd⁺ strain. This is comparable to the level of UV-mutability in the mfd⁺ strain in the presence of caffeine, an inhibitor of dimer excision. UV-mutability, prophage induction and Weigle reactivation of irradiated λ phage occur to a greater extent at low UV doses (10–50 erg/mm²) in the mfd strain compared to the mfd⁺ strain. We propose that the slow excision repair in the mfd mutant results in a shift in the induction threshold for these UV-inducible functions toward lower UV doses.     

异鳔鳅年龄结构、生长特性与生活史类型

2010年10月至2011年7月在长江上游宜宾至江津江段采集405尾异鳔鳅鮀(Xenophysogobio boulengeri)样本。以耳石作为主要年龄鉴定材料,研究了异鳔鳅鮀的年龄结构和生长特征,采用模糊聚类分析法推断了异鳔鳅鮀的生活史类型。结果表明:长江上游异鳔鳅鮀由4个年龄组组成,其中优势年龄组为1～2龄(占83.70%);体长(L)与体重(W)关系为W=5×10-6L3.294 3(r2=0.941 3,n=405,F=6 040.22,P0.01),体长与耳石半径(R)的关系为L=0.000 8R2-0.168 4R+42.504(r2=0.836 5,n=399,F=399.20,P0.01);von Bertalanffy方程描述的生长方程为Lt=179.49(1-e-0.249 8(t+0.289 8))、Wt=133.18(1-e-0.249 8(t+0.289 8))3.294 3,体重生长拐点年龄为4.48龄;异鳔鳅鮀属于r-选择类型鱼类。当前长江上游异鳔鳅鮀群体生长特征较2001～2002年已发生变化,因此有必要采取保护措施。     

The Levels of a Universally Conserved tRNA Modification Regulate Cell Growth

N⁶-Threonylcarbamoyl-adenosine (t⁶A) is a universal modification occurring at position 37 in nearly all tRNAs that decode A-starting codons, including the eukaryotic initiator tRNA (tRNA_i^Met). Yeast lacking central components of the t⁶A synthesis machinery, such as Tcs3p (Kae1p) or Tcs5p (Bud32p), show slow-growth phenotypes. In the present work, we show that loss of the Drosophila tcs3 homolog also leads to a severe reduction in size and demonstrate, for the first time in a non-microbe, that Tcs3 is required for t⁶A synthesis. In Drosophila and in mammals, tRNA_i^Met is a limiting factor for cell and animal growth. We report that the t⁶A-modified form of tRNA_i^Met is the actual limiting factor. We show that changing the proportion of t⁶A-modified tRNA_i^Met, by expression of an un-modifiable tRNA_i^Met or changing the levels of Tcs3, regulate target of rapamycin (TOR) kinase activity and influences cell and animal growth in vivo. These findings reveal an unprecedented relationship between the translation machinery and TOR, where translation efficiency, limited by the availability of t⁶A-modified tRNA, determines growth potential in eukaryotic cells.     

Essentiality of threonylcarbamoyladenosine (t6A), a universal tRNA modification,in bacteria

Threonylcarbamoyladenosine (t⁶A) is a modified nucleoside universally conserved in tRNAs in all three kingdoms of life. The recently discovered genes for t⁶A synthesis, including tsaC and tsaD, are essential in model prokaryotes but not essential in yeast. These genes had been identified as antibacterial targets even before their functions were known. However, the molecular basis for this prokaryotic‐specific essentiality has remained a mystery. Here, we show that t⁶A is a strong positive determinant for aminoacylation of tRNA by bacterial‐type but not by eukaryotic‐type isoleucyl‐tRNA synthetases and might also be a determinant for the essential enzyme tRNA^Ile‐lysidine synthetase. We confirm that t⁶A is essential in Escherichia coli and a survey of genome‐wide essentiality studies shows that genes for t⁶A synthesis are essential in most prokaryotes. This essentiality phenotype is not universal in Bacteria as t⁶A is dispensable in Deinococcus radiodurans, Thermus thermophilus, Synechocystis PCC6803 and Streptococcus mutans. Proteomic analysis of t⁶A^? D. radiodurans strains revealed an induction of the proteotoxic stress response and identified genes whose translation is most affected by the absence of t⁶A in tRNAs. Thus, although t⁶A is universally conserved in tRNAs, its role in translation might vary greatly between organisms.     

Effect of theophylline and Na+ on methionine influx in Na+-depleted intestine

The unidirectional influx of methionine into the brush border epithelium of chicken jejunum has been studied. Tissues leached of Na⁺ transport methionine from a medium devoid of Na⁺ with reduced apparent affinity (K_t) and maximal flux (J_max). Addition of Na⁺ to the medium during a 1-min incubation with substrate, or during a 30-min preincubation, restored K_t but affected J_max slightly. Theophylline was found to maintain J_max in the absence of Na⁺. Essentially complete restoration of K_t and J_max could be attained when theophylline-treated tissue was exposed to Na⁺ for 30 min. Influx from a Na⁺ medium was unaffected by theophylline pretreatment in Na⁺-containing buffer. K_t was increased without an effet upon J_max when influx was studied from choline medium following preincubation in Na⁺.Modifiers of tissue cyclic AMP levels were investigated in conjunction with theophylline. Histamine and carbachol were found to inhibit theophylline-stimulated transport. Secretin was found to stimulate influx in Na⁺-leached tissue, but did not potentiate the theophylline effect. Amino acids in the incubation medium inhibited theophylline-stimulated influx, whereas preloaded lysine or methionine had no effect.The results are interpreted in terms of a model which envisions roles for cellular and external Na⁺ and for cyclic AMP in the activation and regulation of amino acid transport in intestine.     

The Methyl Group of the N6-Methyl-N6-Threonylcarbamoyladenosine in tRNA of Escherichia coli Modestly Improves the Efficiency of the tRNA

tRNA species that read codons starting with adenosine (A) contain N⁶-threonylcarbamoyladenosine (t⁶A) derivatives adjacent to and 3′ of the anticodons from all organisms. In Escherichia coli there are 12 such tRNA species of which two (tRNA_GGU^Thr1 and tRNA_GGU^Thr3) have the t⁶A derivative N⁶-methyl-N⁶-threonylcarbamoyladenosine (m⁶t⁶A37). We have isolated a mutant of E. coli that lacks the m⁶t⁶A37 in these two tRNA_GGU^Thr species. These tRNA species in the mutant are likely to have t⁶A37 instead of m⁶t⁶A37. We show that the methyl group of m⁶t⁶A37 originates from S-adenosyl-l-methionine and that the gene (tsaA) which most likely encodes tRNA(m⁶t⁶A37)methyltransferase is located at min 4.6 on the E. coli chromosomal map. The growth rate of the cell, the polypeptide chain elongation rate, and the selection of Thr-tRNA_GGU^Thr to the ribosomal A site programmed with either of the cognate codons ACC and ACU were the same for the tsaA1 mutant as for the congenic wild-type strain. The expression of the threonine operon is regulated by an attenuator which contains in its leader mRNA seven ACC codons that are read by these two m⁶t⁶A37-containing tRNA_GGU^Thr species. We show that the tsaA1 mutation resulted in a twofold derepression of this operon, suggesting that the lack of the methyl group of m⁶t⁶A37 in tRNA_GGU^Thr slightly reduces the efficiency of this tRNA to read cognate codon ACC.All tRNA species from the three domains, Archaea, Bacteria, and Eucarya, contain modified nucleosides, which are derivatives of the four nucleosides, adenosine, guanosine, cytidine, and uridine. At present, more than 79 different modified nucleosides from the tRNA of various organisms have been characterized (23). Some of these are present in tRNA from only one domain, but a few are present in the same subset of and at the same position in the tRNAs from all three domains (3). One such conserved group of modified nucleosides is the threonylated adenosine (t⁶A) derivatives. These modified adenosines are present adjacent to and 3′ of the anticodon (position 37) in the subset of tRNAs that reads codons starting with A. The universal presence of t⁶A derivatives suggests that these kinds of modifications may have been present in the tRNA of the progenitor, unless a convergent evolution has occurred. This conservation also suggests that the functions of these modified nucleosides may be principally the same in all organisms.In Escherichia coli, the t⁶A37 derivative N⁶-methyl-N⁶- threonylcarbamoyladenosine (m⁶t⁶A37) is present in only two tRNA species, the tRNA_GGU^Thr species, with the same anticodon (20). Threonine is the precursor in the synthesis of t⁶A (10, 32), and in vitro threonylation requires carbonate and ATP (15, 21). Here we show that the methyl group of m⁶t⁶A37 originates from methionine. So far, no mutant deficient in any t⁶A37 derivative has been characterized. As a first step to elucidate the syntheses of these groups of modified nucleosides and their roles in vivo, we have isolated and characterized a mutant deficient in the synthesis of m⁶t⁶A37. We show that the tsaA gene most likely encodes the tRNA(m⁶t⁶A37)methyltransferase that transfers a methyl group from S-adenosylmethionine (AdoMet) to the two tRNA_GGU^Thr species containing the t⁶A moiety. The tsaA gene was localized to the 4.6 min site on the E. coli chromosome. We also show that the methyl group of m⁶t⁶A37 slightly improves the translational efficiency of the two tRNA_GGU^Thr species.     

Human Cells Have a Limited Set of tRNA Anticodon Loop Substrates of the tRNA Isopentenyltransferase TRIT1 Tumor Suppressor

Human TRIT1 is a tRNA isopentenyltransferase (IPTase) homologue of Escherichia coli MiaA, Saccharomyces cerevisiae Mod5, Schizosaccharomyces pombe Tit1, and Caenorhabditis elegans GRO-1 that adds isopentenyl groups to adenosine 37 (i6A37) of substrate tRNAs. Prior studies indicate that i6A37 increases translation fidelity and efficiency in codon-specific ways. TRIT1 is a tumor suppressor whose mutant alleles are associated with cancer progression. We report the systematic identification of i6A37-containing tRNAs in a higher eukaryote, performed using small interfering RNA knockdown and other methods to examine TRIT1 activity in HeLa cells. Although several potential substrates contained the IPTase recognition sequence A36A37A38 in the anticodon loop, only tRNA^SerAGA, tRNA^SerCGA, tRNA^SerUGA, and selenocysteine tRNA with UCA (tRNA^[Ser]SecUCA) contained i6A37. This subset is a significantly more restricted than that for two distant yeasts (S. cerevisiae and S. pombe), the only other organisms comprehensively examined. Unlike the fully i6A37-modified tRNAs for Ser, tRNA^[Ser]SecUCA is partially (∼40%) modified. Exogenous selenium and other treatments that decreased the i6A37 content of tRNA^[Ser]SecUCA led to increased levels of the tRNA^[Ser]SecUCA. Of the human mitochondrion (mt)-encoded tRNAs with A36A37A38, only mt tRNAs tRNA^SerUGA and tRNA^TrpUCA contained detectable i6A37. Moreover, while tRNA^Ser levels were unaffected by TRIT1 knockdown, the tRNA^[Ser]SecUCA level was increased and the mt tRNA^SerUGA level was decreased, suggesting that TRIT1 may control the levels of some tRNAs as well as their specific activity.