Impairment of reovirus mRNA methylation in extracts of interferon-treated Ehrilich ascites tumor cells: further characteristics of the phenomenon

We reported earlier that the methylation of unmethylated reovirus mRNA (reo mRNAU) by the cellular methylating enzymes is impaired in extracts of uninfected, interferon-treated Ehrilich ascites tumor cells (S30INT). We find now that after the methylation of reo mRNAU has stopped in S30INT, the RNA can be reisolated and further methylated in an extract of control cells (S30C). Thus the impairment of methylation in S30INT cannot be due to cleavage or irreversible inactivation of reo mRNAU. Freshly added reo mRNAU can be methylated in S30INT in which the methylation of previously added reo mRNAU has stopped. This indicates that the impairment is due to the depletion of S-adenosylme thionine (the methyl donor), the accumulation of S-adenosylhomocysteine (an inhibitor of methylation), or the irreversible inactivation of reo mRNAU. Freshly added reo mRNAU can be methylated in S30INT in which the methylation of previously added reo mRNAU has stopped. This indicates that the impairment is not due to the depletion of S-adenosylmethionine (the methyl donor), the accumulation of S-adenoxylhomocysteine (an inhibitor of methylation), or the irreversible inactivation of the methylating enzymes. It may be due, however, to the unavailability of reo mRNAU for methylation. The extent of the impairment of reo mRNAU methylation in S30INT decreases with an increasing concentration of reo mRNAU but is not affected by added poly (U), ribosomal RNA, or encephalomyocarditis virus RNA (an mRNA that is probably not capped or methylated at its 5' end). The methylation of reo mRNAU is also impaired in an extract from cells that have not been treated with interferon but with the interferon inducer poly(I) - poly(C). The inhibitor is apparently a macromolecule that is inactivated during incubation. It decreases the methylation at the 7 position of the 5' terminal guanylate residue. In vitro, the rate of reo mRNA synthesis by reovirus cores in the presence of S30INT is the same as in the presence of S30C. However, the methylation of the de novo synthesized reo mRNA by the core-associated methylating enzyme(s) in vitro is inhibited by S30INT but not by S30C. The relevance of these phenomena to the inhibition of reovirus replication in interferon-treated cells remains to be established.

Potential inhibitors of S-adenosylmethionine-dependent methyltransferases. 4. Further modifications of the amino and base portions of S-adenosyl-L-homocysteine

Structural analogues of S-adenosyl-L-homocysteine (L-SAH), with modifications in the amino acid or base portions of the molecule, have been synthesized and evaluated for their abilities to inhibit the transmethylations catalyzed by catechol O-methyltransferase (COMT), phenylethanolamine N-methyltransferase (PNMT), histamine N-methyltransferase (HMT), hydroxyindole O-methyltransferase (HIOMT), and indoleethylamine N-methyltransferase (INMT). From these studies some interesting and potentially useful differences in the structural features of L-SAH needed to produce maximal binding to these methyltransferases were detected. This paper provides evidence that 8-azaadenosyl-L-homocysteine is a potent and selective inhibitor of HIOMT, whereas N6-methyladenosyl-L-homocysteine and N6-methyl-3-deazaadenosyl-L-homocysteine are selective inhibitors in INMT. In contrast, it was found that S-tubercidinyl-L-homocysteine was a fairly potent, but nonselective inhibitor of all of the methyltransferases studied. The differences and the similarities in the requirements for the binding of SAH to methyltransferases which were detected in this study and earlier studies from our laboratory, are described. The possibilites of utilizing differences in binding requirements for the design of SAH analogues as specific inhibitors of methyltransferases are discussed.

Analogues of S-adenosylhomocysteine as potential inhibitors of biological transmethylation. Synthesis of analogues with modifications at the 5'-thioether linkage

The synthesis of S-adenosylhomocysteine analogues, in which the 5'-thioether linkage is replaced by an oxygen or nitrogen isostere, has been investigated. These compounds were disigned to be resistant to enzyme-catalyzed hydrolytic cleavage of the 5'-substituent. The amine analogue Id and two amide analogues 20 were prepared via alkylation or acylation of appropriately blocked adenosine derivatives. These new analogues were evaluated as inhibitors of catechol O-methyltransferase and tRNA methylases and found to have poor inhibitory activity.

Ribonuclease H of calf thymus: substrate specificity, activation, inhibition

When the action of highly purified specimens of ribonuclease H (hybrid nuclease; RNA-DNA hybrid ribonucleotidohydrolase; EC 3.1.4.34) of calf thymus on a wide selection of homopolymer hybrids was studied, the extent, and even the occurrence, of hydrolysis was found to be governed by the interplay of several factors: the composition of the ribo strand, the length of the deoxyribo strand, and the nature of the activating metal. Mn2+ activates the enzymic cleavage of all hybrid combinations, Mg2+ only of those containing purine ribo strands, Co2+ only of poly(A) hybrids. A 1:1 hybrid of phage f1 DNA and RNA is, however, split in the presence of any of these activators. Hybrids with deoxyribo tetranucleotides can still be cleaved, but not with dinucleotides. The behavior of hybrids containing covalently linked runs of ribo and deoxyribopolynucleotides was studied with the hybrid poly(dT)-poly(A)7-(dA)X]. This hybrid is attacked by ribonuclease H so that the bulk of the resulting poly(dA) still retains one covalently linked riboadenylic acid end group, whereas a small proportion carries a ribo dinucleotide. Inhibition studies showed that ribonuclease H is inactivated irreversibly by pretreatment with S-adenosylmethionine at 35 degrees, but not at 0 degrees. S-Adenosylhomocysteine also is inhibitory, but not irreversibly; also it is essentially limited to the inhibition of the cleavage of purine ribo strands. When the enzyme is exposed simultaneously to both inhibitors, irreversible inactivation is diminished considerably.

[New studies on inhibition of tRNA N2 guanine methyltransferase by S-adenosyl-homocysteine and S-adenosyl-methionine analogs]

New structural analogs of S-adenosyl homocysteine (SAH) 1-9, 11-14, 19-21) and of S-adenosyl methionine (15-18) have been tested as inhibitors of a N-2 guanine methyltransferase extract from rabbit liver with E. coli B tRNA as substrate. The sulfonium compounds (mixture of +/- diastereoisomers) are more inhibitory than the sulfide derivatives but less inhibitory than SAH itself. The replacement of the aminoacid chain in SAH by various alphatic radicals leads to a correlation between their bulk and the size of the enzymatic site. The monosubstitution of N-6 amino group does not affect significantly the inhibitory effect, which is completely canceled by the disubstitution of N-6.

Modification of RNA by mRNA guanylyltransferase and mRNA (guanine-7-)methyltransferase from vaccinia virions

A purified enzyme system isolated from vaccinia virus cores has been shown to modify the 5' termini of viral mRNA and synthetic poly(A) and poly(G) to form the structures m7G(5')pppA- and m7G(5')pppG-. The enzyme system has both guanylyltransferase and methyltransferase activities. The GTP:mRNA guanylyltransferase activity incorporates GMP into the 5' terminus via a 5'-5' triphosphate bond. The properties of this reaction are: (a) of the four nucleoside triphosphates only GTP is a donor, (b) mRNA with two phosphates at the 5' terminus is an acceptor while RNA with a single 5'-terminal phosphate is not, (c) Mg2+ is required, (d) the pH optimum is 7.8, (e) PP1 is a strong inhibitor, and (f) the reverse reaction, namely the formation of GTP from PP1 and RNA containing the 5'-terminal structure G(5')pppN-, readily occurs. The S-adenosylmethionine:mRNA(guanine-7-)methyltransferase activity catalyzes the methylation of the 5'-terminal guanosine. This reaction exhibits the following characteristics: (a) mRNA with the 5'-terminal sequences G(5')pppA- and G(5')pppG- are acceptors, (b) only position 7 of the terminal guanosine is methylated; internal or conventional 5'-terminal guanosine residues are not methylated, (c) the reaction is not dependent upon GTP or divalent cations, (d) optimal activity is observed in a broad pH range around neutrality, (e) the reaction is inhibited by S-adenosylhomocysteine. Both the guanylyltransferase and methyltransferase reactions exhibit bisubstrate kinetics and proceed via a sequential mechanism. The reactions may be summarized: (see article).

Ribosome binding to reovirus mRNA in protein synthesis requires 5' terminal 7-methylguanosine

Purified reovirus synthesizes in vitro a mixture of mRNA molecules that contain 5' terminal structures of the type ppG..., GpppG..., and m7GpppGm, the relative amount of each depending upon the presence of S-adenosylmethionine (SAM) or S-adenosylhomocysteine (SAH) in the transcription incubation mixture. Reovirus mRNAs containing 5' termini of the type ppG... and GpppG... can be specifically modified by wheat germ extracts in the presence of SAM to yield the structure m7GpppG...(Muthukrishnan et al., 1975). The mRNA methylase activity is ribosome-independent and is recovered almost entirely in the high speed supernatant fraction of wheat germ extracts. Its activity is inhibited by aurintricarboxylic acid. Ribosome binding experiments with reovirus mRNA and wheat germ extract indicate that only those mRNA molecules containing 5' terminal m7G are capable of participating in the initiation of protein synthesis and subsequent polysome formation. mRNA with unmethylated 5' termini do not form a complex with 40S ribosomal subunits. Discrimination between unmethylated and methylated reovirus mRNA, active in protein synthesis, apparently occurs at or before the formation of 40S-mRNA complexes. T1 or pancreatic RNAase digestion of methylated mRNA bound to 80S ribosomes yields 5' terminal fragments of apparent chain lengths about 32 and 36 nucleotides, respectively. A portion of the T1 RNAase-resistant fragments rebinds to ribosomes to form a nuclease-resistant complex. In contrast, the shorter 5' terminal oligonucleotide m7GpppGmpCpUp(Np)3Gp derived by RNAase T1 digestion of purified reovirus mRNA does not bind to ribosomes. The results suggest that 5' terminal m7G may function as the primary recognition signal for ribosome binding and that wheat germ ribosomes bind very close to the 5' termini of some of the species of methylated reovirus mRNAs.

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.

Methylation of messenger RNA of Newcastle disease virus in vitro by a virion-associated enzyme

Purified Newcastle disease virus contains an enzyme that incorporates the methyl group from S-adenosyl-L-methionine into RNA synthesized in vitro by the virion-associated RNA polymerase (RNA nucleotidyltransferase). Incorporation of radioactivity from S-adenosyl-L-[methyl-3H]methionine was totally dependent upon RNA synthesis. The methylation reaction was completely inhibited by S-adenosyl-L-homocysteine, suggesting the transfer of only the methyl group of S-adenosyl-methionine to RNA products. Velocity sedimentation and hybridization of the in vitro product RNA indicated that both [3H]methyl and [32P]GMP labels resided in single-stranded 18S RNA molecules which were virus specific. Approximately 1 to 2 methyl groups were incorporated per RNA molecule. DEAE-cellulose chromatography of product RNA after alkaline hydrolysis suggested that the 5' terminus was the site of methylation.