Stimulation of soluble ribonucleic acid methylase activity by polyamines

Specificity of a nucleolar 2'-O-methyltransferase for RNA substrates

In this study we examined the specificity of a nucleolar 2'-O-methyltransferase isolated from nucleoli of Ehrlich ascites tumor cells. The nucleolar methyltransferase was capable of methylating each of the four nucleosides of RNA, however, the level of methylation at a particular nucleoside varied with the type of RNA. Both kinetic analysis and the stimulation of methylation by polyamines suggested that the structure of RNA was critical in influencing the discrimination and apparent specificity of nucleolar 2'-O-methyltransferase.

Effect of polyamines on in vitro reconstitution of ribosomal subunits

The effect of polyamines on in vitro reconstitution of Escherichia coli 30S and 50S ribosomal subunits has been studied. Spermidine stimulated the reconstitution of 30S particles from 16S rRNA lacking the methyl groups on two neighboring adenines and total proteins of 30S subunits at least 1.6-fold. The reconstitution of 30S particles from normal 16S rRNA and total proteins of 30S subunits exhibited only slight spermidine stimulation. However, the optimal Mg2+ concentration of the reconstitution was decreased from 20 mM to 16 mM in the presence of 3 mM spermidine. In the absence of spermidine the assembly of 30S particles from normal 16S rRNA was more rapid than the assembly from 16S rRNA lacking the methyl groups on two neighboring adenines. The reconstitution of 50S particles from 23S and 5S rRNA and total proteins of 50S subunits was not influenced greatly by spermidine. Gel electrophoresis results, from reconstitution experiments of 30S particles from 16S rRNA lacking the methyl groups on two neighboring adenines and total proteins of 30S subunits, showed that the assembly of S1 and S9 proteins to 23S core particles was stimulated by spermidine during reconstitution. The relationship of polyamine effects on in vitro ribosome assembly from its constituents to in vivo ribosome assembly is discussed. The reconstitution of Bacillus subtilis 30S particles from 16S rRNA and total proteins of 30S subunits was also stimulated approximately 1.3-fold by 3 mM spermidine.

Excretion of polyamines by children with leukemia during chemotherapy

Evidence is presented showing a relation between polyamine concentration and the methylation of tRNA in vitro and in vivo. Polyamine excretion in urine of children with ALL and AML is slightly elevated before the commencement of chemotherapy. Immediately thereafter, excretion of acetylputrescine increases drastically over a period of at least 30-60 days. The elevation seems to be caused by different factors, e.g., destruction of tumor cells, induction of ornithindecarboxylase, and cell recovery after termination of chemotherapy. Acetylspermidine excretion also increases 3-4 days after the beginning of chemotherapy. A positive correlation exists between leukocyte counts and excretion of acetylspermidine. The ratio of acetylspermidine excretion at days 3-4 of the therapy to that before commencement of chemotherapy could be an indicator of response to the therapy.

Effects of cortisol on tRNA methylase activities in rat mammary carcinoma

Mammary carcinomas induced in rats by DMBA were divided into three types: I, hard proliferating tumors; II, tumors presenting from an early stage the first signs of cystic degeneration; III, lactating tumors. In all three types, cortisol reduced the protein content by 26%-30%. The already high tRNA methyltransferase activity in type I increased by 200% after cortisol treatment. Hormonal treatment of type II increased the previously reduced control methyltransferases by 37%. In the type III lactating tumors, the total tRNA methyltransferases were inhibited by 35% after cortisol treatment. The methyltransferases of types I and II were separated chromatographically into seven analogous peaks, while the enzymes from type III presented a modified pattern. In each case, cortisol treatment affected the activities of several methyltransferases simultaneously without obvious specificity.

Regulation of tRNA methyltransferase activities by spermidine and putrescine. Inhibition of polyamine synthesis and tRNA methylation by alpha-methylornithine or 1,3-diaminopropan-2-ol in Dictyostelium

Inhibitors of polyamine synthesis (alpha-methylornithine and 1,3-diaminopropan-2-ol) were used to study the relationship between polyamine synthesis and specific methylations of tRNA in Dictyostelium discoideum during vegetative growth. Polyamine concentrations were found to be 10 mM for putrescine, 1.6 mM for spermidine and 7 mM for 1,3-diaminopropane throughout the growth stage. On treatment of growing amoebae with alpha-methylornithine or with 1,3-diaminopropan-2-ol (each at 5 mM), the syntheses of putrescine, spermidine and 1,3-diaminopropane were arrested within 4h. After polyamine synthesis had ceased, the incorporation of methyl groups into tRNA was considerably decreased under conditions that had no effect on the incorporation of uridine into tRNA, or on net syntheses of protein and of DNA. The following nucleosides in tRNA were concerned: 1 methyladenosine, 5-methylcytidine, 7-methylguanosine, 2-methylguanosine, N2N2-dimethylguanosine and 5-methyluridine (ribosylthymine). The corresponding tRNA methyltransferases, determined in Mg2+-free enzyme extracts, proved to be inactive unless polyamines were added. Putrescine and/or spermidine at concentrations of 10 mM or 1-2 mM respectively stimulate the transmethylation reaction in vitro to a maximal rate and to an optimal extent at exactly the same concentrations as found in vegetative cells. In contrast, 1,3-diaminopropane, which is formed from spermidine, does not affect the methylation of tRNA in vitro at physiological concentrations. Putrescine and/or spermidine stabilize the tRNA methyltransferases in crude extracts in the presence but not in the absence of the substrate tRNA. The results support the view that S-adenosylmethionine-dependent transmethylation reactions can be regulated by alterations of polyamine concentrations in vivo.

Stimulation of glycosaminoglycan sulfotransferase from chick embryo cartilage by basic proteins and polyamines

A soluble glycosaminoglycan sulfotransferase (3'-phosphoadenylylsulfate:chondroitin 4'-sulfotransferase, EC 2.8.2.5) from chick embryo cartilage has been prepared free from endogenous acceptor. The reaction with this enzyme preparation was stimulated by basic proteins and polyamines, the degree of stimulation being dependent on the chemical nature of both basic compounds and acceptor glycosaminoglycans. A maximum stimulation was obtained when protamine (basic compound) and chondroitin (acceptor) were involved in the reaction mixture at a molar ratio of protamine to repeating disaccharide units of chondroitin, 1:100. The stimulation of sulfotransferase activity by basic substances was much higher than that by Mn2+. However, increasing the Mn2+ concentration immediately reduced the stimulation by basic substances. The Km value for 3'-phosphoadenosine 5'-phosphosulfate of the sulfotransferase, when chondroitin was used as acceptor, was 1 . 10(-6) M in the presence of 25 microgram/ml protamine, compared to 2 . 10(-5) M in the absence of protamine. These observations indicate that the basic proteins and polyamines may interact with acceptor polysaccharide, thereby causing an increase in the affinity of the enzyme toward 3'-phosphoadenosine 5'-phosphosulfate.

S-adenosylhomocysteine analogues as inhibitors of specific tRNA methylation

Of 17 base- or amino acid-modified analogues of S-adenosylhomocysteine, six were found to produce at least 50% inhibition of the activity of an unfractionated tRNA methyltransferase extract at concentrations of 200 micron. The inhibitory effects of these six analogues on five purified rat liver tRNA methyltransferases were examined. The purified enzymes differed greatly in their sensitivity to the analogues. Ki values for the inhibitory analogues were determined for the three most highly purified methyltransferases. The kinetic analyses indicated that inhibition is competitive for nearly all enzyme/inhibitor combinations. The Ki values for good enzyme/inhibitor pairs were in the range of 0.11--2 micron. Each analogue appears to inhibit one methylation more strongly than others; e.g. the Ki values obtained for N6-methyl-S-adenosyl-L-homocysteine are approx. 0.4 micron for guanine-1 tRNA methyltransferase, 6 micron for adenine-1 tRNA methyltransferase and 100 micron for N2-guanine tRNA methyltransferase I. Structural features which are important for inhibitory activity are presence of a terminal amino group on the amino acid and the presence of adenosine rather than any other base. Ring nitrogens, a terminal carboxyl group and conformation at the asymmetric carbon appear to be important for some but not all of the tRNA methyltransferases examined.

Stringent regulation of the synthesis of a transfer ribonucleic acid biosynthetic enzyme: transfer ribonucleic acid(m5U)methyltransferase from Escherichia coli

This paper describes the regulation of a transfer ribonucleic acid (tRNA) biosynthetic enzyme, the tRNA(m5U)methyltransferase (EC 2.1.1.35). This enzyme catalyzes the formation of 5-methyluridine (m5U, ribothymidine) in all tRNA chains of Escherichia coli. Partial deprivation of charged tRNAVal can be imposed by shifting strains carrying a temperature-sensitive valyl-tRNA ligase from a permissive to a semipermissive temperature. By using two such strains differing only in the allelic state of the relA gene, it was possible to show the tRNA(m5U)methyltransferase to be stringently regulated. Upon partial deprivation of charged tRNAVal, the differential rate of tRNA(m5U)methyltransferase synthesis was found to decrease in a strain with stringent RNA control (relA+), whereas it increased in the strain carrying the relA allele. This increase of accumulation of tRNA(m5U)methyltransferase activity required protein synthesis. Thus, when tRNA is partially uncharged in the cell, the relA gene product influences the expression of tRNA(m5U)methyltransferase gene.

Effects of amines on macromolecular methylation

Histone methylation by extracts of rat brain or liver was inhibited and tRNA methylation stimulated by the addition of a number of naturally occurring polyamines. The effect was age independent although the methylase activities are highly age-related. Spermine and/or histamine stimulated methylation of cytosine and adenine to a far grease activity, were more sensitive to inhibition by adenosine than were liver extracts. Adenosine inhibited the methylation of guanine to a greater extent than of cytosine or adenine. Methylation of both tRNA and histone by liver enzyme was inhibited by L-dopa, dopamine and epinephrine. Methylation by brain enzyme was also blocked, but less extensively. The response of liver extracts to these catecholamines was highly age-related. The phenolic amines, octopamine, synephrine, serotonin and tyramine, stimulated tRNA methylation slightly while inhibiting histone methylation by both liver and brain extracts and these effects showed no age dependency. Analysis of the data suggests that most of these compounds do not act by competing for the available S-adenosylmethionine.

Studies on the mechanism of inhibition of tRNA methylation by 3,4-dihydroxyphenylethylamine

A Lineweaver-Burk analysis of a kinetic study of tRNA methylation by a 30-50% (NH4)2SO4 fraction from a weanling rat liver extract showed competitive inhibition with a Km for S-adenosylmethionine = 0.66 - 10(-6) M and a Ki for 3,4-dihydroxyphenylethylamine (dopamine) = 4 - 10(-5) M. The dopamine-inhibited methylation of tRNA appears to be linear with time. Rapid-flow dialysis studies indicated a S-adenosylmethionine binding constant of 0.65 - 10(-6) M. Dopamine appeared to interfere with the binding of S-adenosylmethionine to the weanling rat liver protein preparation but did not affect the binding of S-adenosylmethionine to protein in several systems in which dopamine did not inhibit tRNA methylase activity.

The methylation of tRNA

The methylation of tRNA is a post-transcriptional modification which is achieved by specific enzymes, the tRNA methylases, with S adenosylmethionine as a methyl donor. The level and pattern of methylation are characteristic of the tRNA species and origin. Abnormally methylated tRNAs have been obtained, in vivo and in vitro, by a variety of methods, and their properties have been studied. The tRNA methylases are found in all cells and tissues. Their activity varies with the differentiation state of the cells, and under the influence of many internal and external factors ; it is especially elevated in embryonic and cancerous tissues. These enzymes are very unstable, and none of them has been purified to homogeneity. We present here their known properties and we propose a theory concerning their specificity. Finally, after reviewing the few available experimental data, we discuss the current hypotheses and speculations about the roles and functions of tRNA methylation.

Rate of tritium labeling of specific purines in relation to nucleic acid and particularly transfer RNA conformation

The kinetics of the incorporation of tritium into the C-8 positions of purine units in nucleic acids has been studied. The polymers investigated include poly(A), poly(A): poly (U) duplex, a double-stranded viral RNA, tRNA, and DNA. In the random coil state, the kinetics of incorporation of tritium into the purine sites of the polymers are identical with those for the corresponding purine mononucleotides. When the nucleic acids are in their native conformations, however, the purine labeling rates are reduced below that expected for the free mononucleotides. The magnitude of the effect is remarkably dependent upon the particular nucleic acid. For example, at 37 degrees C the purines in double-stranded DNA label at a rate two- to threefold slower than the corresponding mononucleotides, but in a double-stranded viral RNA, a 30- to 40-fold effect is found. The data suggest a strong influence of microscopic helix structure on the rate of tritium incorporation. First-order rate constants for the exchange of tritium into specific purine sites in yeast tRNAPhe were also determined. This was done by partially labeling the nucleic acid in tritiated water, and subsequently removing free and loosely bound tritium. Under conditions where exchange-out does not occur, the nucleic acid was digested with specific nucleases; chromatographic separation then enabled specific activities of purines from specific sites to be obtained. The rate constants for these sites show a large variation. They are markedly reduced for those residues occurring in cloverleaf helical sections and, in certain cases, for those known from crystallographic data to be involved in tertiary interactions. As examples of bases that can participate in tertiary interactions, the crystal structures show A14 and G15 in special base-pairing arrangements. Both purines (A14 and G15) occur in single-stranded sections of the cloverleaf; both show markedly reduced C-8 hydrogen-exchange rates. On the other hand, rate constants for bases and regions known to be on the outside of the moleculesuch as the anticodon loop and the 3' terminusāre perturbed the least. In one instance, a base in the dihydrouridine loop believed to be involved in tertiary interactions, according to crystallographic studies, incorporates tritium as if it were relatively unperburbed by the tRNA structure. The structural interactions of this base may be partially or completely broken at 37 degrees C in solution.

tRNA methyltransferases from rat liver. Differences in response of partially purified enzymes to polyamines and inorganic salts

Three tRNA methyltransferases, purified from rat liver, have been compared for their activity in the presence of various amines and Mg2+. The enzymes differ with respect to the ion which permits maximal activity; they also differ with respect to the concentration of a given ion necessary for maximal activity. The methyltransferase which forms N2-methylguanine in the region between the dihydrouridine loop and the acceptor stem (2mG I), when assayed using purified tRNA as substrate, shows high activity with 3--5 mM sperimidine or 20 mM putrescine and significantly lower rates of methylation with 200--350 mM ammonium acetate or 1--10 mM magnesium acetate. The enzyme responsible for forming N2-methylguanine between the dihydrouridine and anticodon loops (2mG II) works well in the presence of 0.2--0.5 mM spermidine, 10 mM putrescine or 200--300 mM ammonium acetate and shows slightly lower activity with 1 mM magnesium acetate. The optimal conditions for assaying 1-adenine methyltransferase (1mA) with purified tRNAs are either 200--300 mM ammonium acetate or 30 mM putrescine; spermidine is slightly less effective and magnesium acetate permits less than 25% of maximal activity. The addition of 10 mM Mg2+, in combination with polyamines or NH4+, depresses slightly the activity of the guanine methyltransferases but completely abolishes the polyamine or ammonium-stimulated activity of the adenine methyltransferase. When unfractionated (Escherichia coli) tRNA is used as substrate, the concentrations of polyamines required for optimal methyltransferase activity are increased but the patterns of response of the three enzymes do not differ significantly from those obtained with purified tRNA substrates. Based on the studies with these three enzymes, unfractionated tRNA and 40 mM putrescine should provide the most reliable system for detecting methylating activity if the nature of the tRNA methyltransferase is unknown.

Transfer RNA methyltransferase activity in paramecium aurelia

The tRNA methyltransferases from Paramecium aurelia were investigated. The effects of varying the Mg2+ and NH4+ concentrations, pH, and temperature on the methylation of Escherichia coli B tRNA using extracts from P. aurelia were determined. Optimum tRNA methyltransferase activity was observed at pH 7.8 and 37 degrees C. The Mg2+ optimum occurred at 0.66 mM in the absence of NH4+ while the NH4+ optimum occurred at 100 mM in the absence of Mg2+. Analysis of the bases methylated in (E. coli B) tRNA by extracts of P. aurelia showed the presence of 1-methyladenine, 1-methylguanine, N2-methylguanine, N2,N2-dimethylguanine and methylated pyrimidine nucleotides. In comparison, an analysis of the in vivo methylation of tRNA from P. aurelia showed the presence of 1-methyladenine, 6-methyladenine, 6,6-dimethyladenine, 1-methylguanine, N2-methylguanine, N2,N2-dimethylguanine, 7-methylguanine, and methylated pyrimidine nucleotides. The pattern of methylation of tRNA in P. aurelia is similar to that observed in other eukaryotes.

Ornithine decarboxylase activity in relation to DNA synthesis in mouse interfollicular epidermis and hair follicles

The relationship between ornithine decarboxylase (L-ornithine carboxylyase, EC 4.1.1.17) activity and DNA synthetic activity was studied in mouse epidermis. Interfollicular epidermis and hair follicles were investigated separately. It was found that, in hair follicles, the variations of DNA replicative activity, which are reflected in the cyclic growth of hair, are paralleled by corresponding changes in ornithine decarboxylase activity. In both interfollicular epidermis and hair follicles, stimulation of DNA synthetic activity by plucking of hair induced a rapid and marked increase in ornithine decarboxylase activity. The relationship of steady-state and induced ornithine decarboxylase activity to DNA synthetic activity was compared in hair follicles and interfollicular epidermis. A correlation between the activity of this enzyme and DNA replication was found thereby in each of these tissues.

S-adenosylhomocysteine inhibition of three purified tRNA methyltransferases from rat liver

Three tRNA methyltransferases from rat liver have been fractionated and purified greater than 100-fold. These enzymes have been examined for their sensitivity to inhibition by S-adenosylhomocysteine (SAH). The methyltransferase which forms m2-guanine in the region between the dihydrouridine loop and the acceptor stem of tRNA (m2-guanine methyltransferase I) is least sensitive to SAH inhibition, with a Ki of 8 muM. The enzyme responsible for forming m2-guanine between the dihydrouridine and anticodon loops (m2-guanine methyltransferase II) has a Ki of 0.3 muM, while m1-adenine methyltransferase shows intermediate sensitivity to SAH (Ki = 2.4 muM). All three methyltransferases have similar Km's for the S-adenosylmethionine substrate (1.5-2.0 muM). These results are consistent with the hypothesis that activity of individual tRNA methyltransferases may be controlled by enzyme systems which alter cellular SAH levels.

A study of various changes in transfer ribonucleic acid methylase activity during adenovirus-12 transformation in vitro

A number of parameters were used to correlate a change in control or expression of tRNA methylase activity and the transformation by adenovirus-12 in vitro. The earliest change observed corresponding to visible morphological changes was a decrease in the tRNA methylase inhibitor(s).

Enzymatic methylations: III. Cadaverine-induced conformational changes of E. coli tRNA fMet as evidenced by the availability of a specific adenosine and a specific cytidine residue for methylation

A partially purified tRNA methylase fraction from rat liver, containing m(2)G- m(1)A- and m(5)C-methylase, was used to study the influence of Mg(++) and of the biogenic polyamine cadaverine on the enzymatic methylation of E.coli tRNA(fMet)in vitro. In presence of 1 or 10 mM Mg(++), guanosine no. 27 was methylated to m(2)G. In 1 mM Mg(++) plus 30 mM cadaverine, guanosine in position 27 and adenosine in position 59 were methylated. In presence of 30 mM cadaverine alone tRNA(fMet) accepted three methyl groups: in addition to guanosine no. 27 and adenosine no. 59 cytidine no. 49 was methylated. In order to correlate tRNA(fMet) tertiary structure changes with the methylation patterns, differentiated melting curves of tRNA(fMet) were measured under the methylation conditions. It was shown that the thermodynamic stability of tRNA(fMet) tertiary structure is different in presence of Mg(++), or Mg(++) plus cadaverine, or cadaverine alone. From the differentiated melting curves and from the methylation experiments one can conclude that at 37 degrees in the presence of Mg(++) tRNA(fMet) has a compact structure with the extra loop and the TpsiC-loop protected by tertiary structure interactions. In Mg(++) plus cadaverine, the TpsiC-loop is available, while the extra loop is yet engaged in teritary structure (G-15: C-49) interactions. In cadaverine alone, the TpsiC-loop and the extra loop are free; hence under these conditions the open tRNA(fMet) clover leaf may be the substrate for methylation. In general, cadaverine destabilizes tRNA tertiary structure in the presence of Mg(++), and stabilizes tRNA(fMet) tertiary structure in the absence of Mg(++). This may be explained by a competition of cadaverine with Mg(++) for specific binding sites on the tRNA. On the basis of these experiments a possible role of biogenic polyamines in vivo may be discussed: as essential components of procaryotic and eucaryotic ribosomes they may together with ribosomal factors facilitate tRNA-ribosome binding during protein biosynthesis by opening the tRNA tertiary structure, thus making the tRNA's TpsiC-loop available for interaction with the complementary sequence of the ribosomal 5S RNA.

Further investigation of the increased transfer ribonucleic acid methylase activity in tumours of the mouse colon

1. Extracts prepared from tumours of the mouse colon induced by 1,2-dimethylhydrazine were considerably more active in catalysing the methylation of tRNA than were extracts from normal colon. The enhanced activity was observed when both unfractionated ;methyl-deficient' tRNA and purified tRNA preparations from yeast and bacteria were used as substrates for methylation. 2. The methylated bases produced in these reactions were identified. There were no differences between the products of the reaction catalysed by extracts of tumour and normal colon. 3. The increased activity of tRNA methylases was not due to the presence in the extracts of stimulatory or inhibitory molecules of low molecular weight such as polyamines or S-adenosylhomocysteine. 4. Other enzymes concerned with tRNA metabolism (RNA polymerase, ATP-tRNA adenylyltransferase, aminoacyl-tRNA ligases) were also increased in activity in the tumour tissue. 5. The extent of methylation of a limiting amount of tRNA was greater when tumour extracts were compared with controls, but in no case was it possible to achieve a stoicheiometric methylation of the purified tRNA preparations used as substrates, and the tumour extracts were not able to methylate tRNA obtained from normal mouse colon. We conclude that the tumours contained greater activities of tRNA methylases but that there was no evidence for changes in the specificity of these enzymes during neoplastic growth.

Inhibition of spermidine formation in rat liver and kidney by methylglyoxal bis(guanylhydrazone)

The effect of methylglyoxal bis(guanylhydrazone), a substance known to inhibit putrescine-dependent S-adenosyl-l-methionine decarboxylase, on polyamine metabolism in liver and kidney was investigated. Almost complete inhibition of the incorporation of putrescine into spermidine was obtained up to 8h after administration of 80mg of methylglyoxal bis(guanylhydrazone)/kg body wt. by intraperitoneal injection. However, by 20h after administration of the inhibitor spermidine synthesis was resumed. Considerable accumulation of putrescine occurred during this period (up to 3 times control concentrations in both tissues), but there was only a slight fall in the spermidine content. These results suggest that the putrescine-activated S-adenosyl-l-methionine decarboxylase plays an essential role in spermidine biosynthesis in rat liver and kidney, and the possibility of using methylglyoxal bis(guanylhydrazone) to study the role of polyamine synthesis in growth is discussed.

Studies of the ethylation of rat liver transfer ribonucleic acid after administration of L-ethionine

1. The ethylated nucleosides present in tRNA isolated from the livers of rats treated with 0.5g of l-ethionine/kg body wt. were investigated. Evidence that this tRNA contained N(2)-ethylguanine, N(2)N(2)-diethylguanine, N(2)-ethyl-N(2)-methylguanine, 7-ethylguanine, two ethylated pyrimidines and ethylated ribose groups was obtained. 2. Ethylation of bacterial tRNA was catalysed by extracts containing tRNA methylases prepared from rat liver by using S-adenosyl-l-ethionine as an ethyl donor, but the rate of ethylation was 20 times less than the rate of methylation with S-adenosyl-l-methionine as a methyl donor. 3. The principal product of such ethylation in vitro was N(2)-ethylguanine and traces of the other ethylated guanines and pyrimidines found in tRNA isolated from rats treated with ethionine in vivo were also found. 1-Ethyladenine was not formed, although 1-methyl-adenine is a major product of methylation of bacterial tRNA by these extracts, and 1-ethyladenine was not present in the rat liver tRNA isolated from ethionine-treated animals. 4. After injection of actinomycin D (15mg/kg body wt.) or l-methionine (1.0g/kg body wt.) before the ethionine, ethylation of tRNA was diminished by about 80% but not completely abolished. Administration of 1-aminocyclopentanecarboxylic acid (2.5g/kg body wt.) to inhibit the formation of S-adenosyl-l-ethionine inhibited ethylation of tRNA by 44%. 5. These results suggest that not all of the ethylation of tRNA that occurs in the livers of rats treated with ethionine is mediated by the action of tRNA methylases acting with S-adenosyl-l-ethionine as a substrate, but that this pathway does occur and accounts for a major part of the observed ethylation. 6. The results are discussed with reference to ethionine-induced hepatocarcinogenesis.

Characterization of S-adenosylmethionine: ribosomal ribonucleic acid-adenine (N 6 -) methyltransferase of Escherichia coli strain B

This study is concerned with the isolation and characterization of the enzyme, S-adenosylmethionine:ribosomal ribonucleic acid-adenine (N(6-)) methyl-transferase [rRNA-adenine (N(6)-) methylase] of Escherichia coli strain B, which is responsible for the formation of N(6)-methyladenine moieties in ribosomal ribonucleic acids (rRNA). A 1,500-fold purified preparation of the species-specific methyltransferase methylates a limited number of adenine moieties in heterologous rRNA (Micrococcus lysodeikticus and Bacillus subtilis) and methyl-deficient homologous rRNA. The site recognition mechanism does not require intact 16 or 23S rRNA. The enzyme does not utilize transfer ribonucleic acid as a methyl acceptor nor does it synthesize 2-methyladenine or N(6)-dimethyladenine moieties. Mg(2+), spermine, K(+), and Na(+) increase the reaction rate but not the extent of methylation; elevated concentrations of the cations inhibit markedly. The purified preparations utilize 9-beta-ribosyl-2,6-diaminopurine (DAPR) as a methyl acceptor with the synthesis of 9-beta-ribosyl-6-amino-2-methylaminopurine. A comparison of the two activities demonstrated that one methyltransferase is responsible for the methylation of both DAPR and rRNA. This property provides a sensitive assay procedure unaffected by ribonucleases and independent of any specificity exhibited by rRNA methyl acceptors.

Transfer ribonucleic acid methylases of bone. Studies on vitamin A and D deficiency

Methods were devised for the assay of tRNA methylases of rat bone. The activities of bone tRNA methylases are similar to those from other mammalian tissues. However, unlike reports on liver methylases, no inhibitors were found in the supernatant fraction from pH5 precipitate of bone extracts. The effects of vitamins A and D on the methylation of tRNA by cell-free extracts of rat bone were studied. Deficiency of either vitamin resulted in a decrease in the rate and extent of tRNA methylation, whereas the administration of vitamin A to hypovitaminotic-A rats and vitamin D to hypovitaminotic-D rats increased the rate and extent of tRNA methylation. These effects appear to be apart from changes in ribonuclease activity or in concentrations of calcium or magnesium. No evidence of inhibitors of tRNA methylases was found in bone extracts from vitamin-deficient rats nor of activators in bone extracts from deficient rats given vitamin A or D. The pattern of tRNA methylation under conditions of vitamin A or D deficiency was not changed, suggesting a generalized cellular deficiency. It was of significance to find that the specificity for methylation of specific bases in tRNA was different after the administration of vitamin A as contrasted with the effects of vitamin D. The possible significance of tRNA methylation to the biochemical action of the vitamins on bone is discussed.