In vitro detection of the enzymatic activity of folate-dependent tRNA (Uracil-54,-C5)-methyltransferase: evolutionary implications

Formation of 5-methyluridine (ribothymidine) at position 54 of the T-psi loop of tRNA is catalyzed by site-specific tRNA methyltransferases (tRNA[uracil-54,C5]-MTases). In eukaryotes and many bacteria, the methyl donor for this reaction is generally S-adenosyl-L-methionine (S-AdoMet). However, in other bacteria, like Enterococcus faecalis and Bacillus subtilis, it was shown that the source of carbon is N(5),N(10)-methylenetetrahydrofolate (CH(2)=THF). Recently we have determined that the Bacillus subtilis gid gene (later renamed to trmFO) encodes the folate-dependent tRNA(uracil-54,C5)-MTase. Here, we describe a procedure for overexpression and purification of this recombinant enzyme, as well as detection of its activity in vitro. Inspection of presently available sequenced genomes reveals that trmFO gene is present in most Firmicutes, in all alpha- and delta-Proteobacteria (except Rickettsiales in which the trmFO gene is missing), Deinococci, Cyanobacteria, Fusobacteria, Thermotogales, Acidobacteria, and in one Actinobacterium. Interestingly, trmFO is never found in genomes containing the gene trmA coding for S-adenosyl-L-methionine-dependent tRNA (uracil-54,C5)-MTase. The phylogenetic analysis of TrmFO sequences suggests an ancient origin of this enzyme in bacteria.

Identification and characterization of archaeal and fungal tRNA methyltransferases

All organisms modify their tRNAs by use of evolutionarily conserved enzymes. Members of the Archaea contain an extensive set of modified nucleotides that were early evidence of the fundamental evolutionary divergence of the Archaea from Bacteria and Eucarya. However, the enzymes responsible for these posttranscriptional modifications were largely unknown before the advent of genome sequencing. This chapter explains methods to identify tRNA methyltransferases in genome sequences, emphasizing the identification and characterization of six enzymes from the hyperthermophilic archaeon Methanocaldococcus jannaschii. We describe methods to express these proteins, purify or synthesize tRNA substrates, measure methyltransferase activity, and map tRNA modifications. Comparison of the archaeal methyltransferases with their yeast homologs suggests that the common ancestor of the archaeal and eucaryal organismal lineages already had extensive tRNA modifications.

A novel methyltransferase required for the formation of the hypermodified nucleoside wybutosine in eucaryotic tRNA

We demonstrate that the product of the yeast open reading frame YML005w is required for wybutosine (yW) formation in the phenylalanine-accepting tRNA of the yeast Saccharomyces cerevisiae. tRNA isolated from a deletion mutant of the YML005w gene accumulates 4-demethylwyosine (ImG-14), a precursor lacking three of the methyl groups of the yW hypermodified base. Since the amino acid sequence of the YML005w gene contains the signature motifs of the seven beta-strand methyltransferases, we now designate the gene TRM12 for tRNA methyltransferase. Using pulse-chase labeling of intact yeast cells with S-adenosyl-L-[methyl-(3)H]methionine, we show that the methylesterified form of yW is metabolically stable.

Preparation, crystallization and preliminary X-ray diffraction analysis of PH1948, predicted RNA methyltransferase from Pyrococcus horikoshii

RNA methyltransferase is responsible for transferring methyl and resulting in methylation on the bases or ribose ring of RNA, which existed widely but mostly remains an open question. A recombinant protein PH1948 predicting RNA methyltransferase from Pyrococcus horikoshii OT3 has been crystallized. The crystals of selenomethionyl PH1948 belong to space group C2, with unit-cell parameters a=207.0 A, b=43.1 A, c=118.2 A, b=92.1 degrees , and diffract X-rays to 2.2 A resolution. The V(M) value was determined to be 2.8 A3/Da, indicating the presence of four protein molecules in the asymmetric unit.

Nuclear Control of Cloverleaf Structure of Human Mitochondrial tRNALys

The evolutionary loss in eukaryotic cells of mitochondrial (mt) tRNA genes and of tRNA structural information in the surviving genes has led to the appearance of mt-tRNAs with highly unusual structural features. One such mt-tRNA is the human mt-tRNALys, which relies on post-transcriptional base modification to achieve correct three-dimensional structure. It has been shown that the in vitro transcript of human mt-tRNALys adopts a particular, non-cloverleaf structure when devoid of modified bases, while the native, fully modified tRNA shows the expected cloverleaf structure. Furthermore, a methyl group at position A9-N1, introduced chemically in an otherwise unmodified mt-tRNALys transcript, was found to induce a stable cloverleaf conformation, raising the question of how the specific methyltransferase recognizes the unmodified transcript. In order to shed light on this unusual case of tRNA maturation, the tRNA modification enzymes contained in protein extracts from either highly purified HeLa cell mitochondria or HeLa cell cytosol were first identified and compared, and then used to analyze the mt-tRNALys. An initial screening for modification activities, using as substrates unmodified in vitro transcripts of tRNA genes with well characterized structures, namely yeast cytosolic tRNAPhe, human cytosolic tRNA3Lys, and human mt-tRNAIle, revealed the presence of nine and 11 modification activities in the mitochondrial and cytosolic protein extracts, respectively, the mitochondrial extract including a tRNA (adenine-9,N1)-methyltransferase activity. The comparison of the level and kinetics of A9-N1 methylation and other secondary modifications in the unmodified, misfolded mt-tRNALys and in a cloverleaf-shaped structural mutant, engineered to adopt the tRNALys cloverleaf structure without post-transcriptional modifications, suggested strongly that the methylation of A9-N1 in tRNALys proceeds via a cloverleaf-shaped intermediate. Therefore, it is proposed that this intermediate is present in the in vitro transcript as part of a dynamic equilibrium, and that the mitochondrial protein extract contains an activity that stabilizes, by secondary modification, such a transient cloverleaf-shaped intermediate. Thus, countering the evolutionary loss of structural information in mt-tRNA genes, the mt-tRNA structure is maintained by a modification enzyme encoded in nuclear DNA.

Location of N2,N2-dimethylguanosine-specific tRNA methyltransferase

Most steps in the maturation of nuclear coded tRNAs occur in the nucleus in eukaryotic cells, but little is known as to the intranuclear location of this RNA maturation pathway. Indirect immunofluorescence experiments using antibody to N2,N2 dimethylguanosine-specific tRNA methyltransferase, a tRNA processing enzyme, and to Nup1p, a nuclear pore protein, show that both locate to the nuclear periphery in wild type cells. Staining of the nuclear membrane is more uniform with anti-Trm1p than the punctate staining observed with antibodies recognizing Nup1p. Biochemical fractionation experiments comparing fractionation of Trm1p with Nup1p, tRNA splicing ligase, and tRNA splicing endonuclease show that Trm1p behaves more like the known peripheral nuclear membrane proteins, Nup1p and tRNA splicing ligase, than like the integral membrane protein, tRNA splicing endonuclease. Cells overproducing Trm1p also concentrate it to the nuclear periphery. Thus, the site(s) of interaction of Trm1p are not easily saturable and are likely to be in excess to Trm1p. Trm1p is shared by mitochondria and the nucleus. Cells transformed with a gene coding Trm1p with a mutant nuclear targeting signal display cytoplasmic staining and an enzyme with increased solubility when compared to the solubility of wild type enzyme. Thus, mutations that prevent the enzyme from entering the nucleus result in an increase in its cytosolic but not mitochondrial concentration suggesting that the mitochondrial/nuclear distribution of Trm1p is not due solely to competition of mitochondrial and nuclear targeting information.

Decreased activities of S-adenosylmethionine synthetase isozymes in hereditary hepatitis in Long-Evans rats

Isozyme patterns of S-adenosylmethionine synthetase have been measured with or without dimethylsulfoxide in liver of LEC rat hereditary hepatitis. The activities of the alpha- and beta-forms are decreased with age after birth, and decreased to a half level of 36 weeks after birth. Concentration of S-adenosylmethionine in the liver is almost a half level of control rat. However, the activity of glycine- and tRNA-methyltransferases in the liver shows no significant change.

N2,N2-dimethylguanosine-specific tRNA methyltransferase contains both nuclear and mitochondrial targeting signals in Saccharomyces cerevisiae

The TRM1 gene of Saccharomyces cerevisiae encodes a tRNA modification enzyme, N2,N2-dimethylguanosine-specific tRNA methyltransferase, which modifies both mitochondrial and cytoplasmic tRNAs. The enzyme is targeted to mitochondria for the modification of mitochondrial tRNAs. Cellular fractionation and indirect immunofluorescence studies reported here demonstrate that this enzyme is also localized to the nucleus. Further, immunofluorescence experiments using strains that overproduce the enzyme show a staining at the periphery of the nucleus suggesting that the enzyme is found in a subnuclear destination near or at the nuclear membrane. There is no obvious cytoplasmic staining in these overproducing strains. Fusion protein technology was used to begin to localize sequences involved in the nuclear targeting of this enzyme. Indirect immunofluorescence studies indicate that sequences between the first 70 and 213 NH2-terminal amino acids of the methyltransferase are sufficient to target Escherichia coli beta-galactosidase to nuclei.

Comparison of methyltransferase activities of pair-fed rats given adequate or methyl-deficient diets

The short-term effects of a lipotrope-deficient (methyl-deficient) diet on tRNA and protein methyltransferase activities have been studied using pair-fed male Fischer rats. The activity of liver N2-guanine tRNA methyltransferase II (NMG2) of animals receiving the methyl-deficient diet (MDD) for 2 weeks was found to be elevated more than 2-fold. This is in agreement with the results of earlier experiments in which the animals were fed ad libitum. These data indicate that the effects of lipotrope-deficient diets on NMG2 activity observed in the earlier studies can be attributed to the nature of the diet, and not to differences in caloric intake. In the same pair-fed animals, very little effect of MDD on the activity of NMG2 of either brain or spleen was observed. In liver, the activity of one of the enzymes that catalyze protein methylation--protein methylase I (S-adenosyl-methionine: protein-arginine N-methyltransferase)--was significantly elevated in response to the lipotrope-deficient diet. In contrast, the activities of protein methylase II (S-adenosylmethionine: protein-carboxy-O-methyltransferase), from control and experimental animals did not differ significantly. Lipotrope-deficient diets are thus seen to induce, within a short period of time, selective changes in the activities of some, but not all, of the liver enzymes that catalyze the methylation of tRNA and protein.

Suppression by methionine and choline of onco-fetal patterns of liver tRNA methyltransferase activities in carcinogen-treated rats

When male Fischer rats were fed Purina chow supplemented with 2% D,L-methionine and 1% choline chloride, the rapid increase in N2-guanine tRNA methyltransferase II (NMG2) activity otherwise seen in response to cancer-promoting doses (0.02% in the diet) of 2-acetylaminofluorene (AAF) was prevented, and the increase in NMG2 activity otherwise caused by carcinogenic doses of AAF (0.06% in the diet) was decreased by 50%. In addition, the return of NMG2 activity to a normal level after completion of a 3-week regimen of 0.06% AAF was accelerated in animals fed the methionine plus choline supplemented diet. As shown earlier in this laboratory, liver tRNA methylating enzyme activities are shifted rapidly to an onco-fetal pattern in rats receiving methyl-deficient diets. This pattern is characterized by selectively elevated NMG2 activity while the activities of other base-specific tRNA methylating enzymes are relatively unchanged. Our combined results indicate that the exogenous supply of methyl groups is a factor in regulating NMG2 activity and can modulate at least one response of animals to carcinogens.

Some properties of the ribosomal RNA methyltransferase encoded by ksgA and the polarity of ksgA transcription

The assay for the ksgA-encoded S-adenosylmethionine--6-N',N'-adenosyl (rRNA) dimethyltransferase has been improved; the gel-filtration molecular weight of partially purified enzyme under two different sets of conditions was found to be 55,000 or 26,000 daltons. We have determined methyltransferase activities in strains where ksgA was brought under the control of the mitomycin C-inducible promoter of the colicin E1 gene. Our studies show that ksgA is transcribed counterclockwise on the Escherichia coli chromosome.

Improved separation of modified nucleosides from tRNA hydrolysates: the patterns of tRNA methylation in rat tissues

A sensitive and reproducible method for the isolation of minor nucleosides derived from tRNA is described. The nucleosides obtained from enzymatic digestion of tRNA are separated into several groups using a QAE Sephadex column and increasing concentrations of boric acid in a step-wise manner. The nucleosides in each group are separated by isocratic elution from a preparative Partisil 10-SCX column and high-performance liquid chromatography at ambient temperature. With this method we have determined the patterns of tRNA methylation in vitro with extracts from rat bone, liver, kidney and adrenal glands. Although different tissues appear to contain the same tRNA methyltransferases, the patterns of methylated nucleosides are different.

Transfer RNA methylase activity and capacity during aflatoxin B1-induced hepatocellular carcinogenesis

Transfer RNA methylase (tRNA methylase) activity and capacity were monitored in whole-rat-liver preparations during the induction of hepatocellular carcinomas by an 8-week aflatoxin B1 dosing regimen that produced minimal toxic effects. Significant phases of elevated tRNA methylase capacity occurred at 6 to 9 weeks (20%) and 24 to 29 weeks (40%). No significant change in tRNA methylase activity was noted over the course of the 55-week experiment. Higher aflatoxin B1 doses, producing acute toxic liver damage, resulted in elevated tRNA methylase activity (50%) and capacity (30%) at least as early as 1 week after dosing. Experiments with individual nodular lesions excised from livers of rats continuously fed a diet containing 2 ppm aflatoxin B1 demonstrated similarly elevated tRNA methylase activities and capacities in hyperplastic (preneoplastic)nodules, with and without histological evidence of carcinoma.

The effect of retinol and retinoic acid on physiological and biochemical changes in retinol-deficient rats

1. The effects of retinol and retinoic acid supplementation of retinol-deficient rats were studied for a variety of metabolic processes shown to be affected by retinol-deficiency. 2. Retinol-deficient rats were found to have decreased body-weight, liver and testes weights, a degeneration of testicular germinal cells, an increased incorporation of labelled choline into liver and testes phospholipids, an increased protein synthetic activity (in vitro) of liver ribosomes, an increased transfer-RNA methyltransferase activity in liver and a decreased activity in testes, an increased DNA content of testicular nuclei, and a decreased uptake of [3-H]thymidine by testicular nuclear DNA. 3. In retinol-deficient rats supplemented for 8 weeks with retinol these changes were reversed, measurements returning to control levels. 4. In retinol-deficient rats supplemented for 8 weeks with retinoic acid all changes were reversed except those in the testes. 5. Testicular signs of retinol deficiency appeared to be delayed when retinoic acid was added to the retinol-deficient diet of weanling rats. This suggests a sparing action of retinoic acid on the rat's utilization of retinol. 6. Suggestions are offered as to why retinoic acid will support growth and development but not spermatogenesis in the rat.