Characterization of polyriboadenylate polymerase from Tetrahymena pyriformis.

Poly(A) polymerase [polyadenylate nucleotidyltransferase, EC 2.7.7.19] was extracted from Tetrahymena pyriformis. The enzyme was demonstrated to be present in three forms by column chromatography on DEAE-cellulose, and they were termed poly(A) polymerase Ia, Ib, and II in order of increasing affinity to the column. The properties of enzymes Ia and Ib were similar except that Ia utilizes poly(A) as a primer rather efficiently. Enzyme II differed from enzymes Ia and Ib not only in elution profile on DEAE-cellulose column chromatography but also in pH and temperature preferences, molecular weight, requirement for divalent cations, sensitivity to salts at high ionic strength, optimal primer concentration, and subcellular localization. The molecular weights of enzymes Ia and Ib measured by gel filtration were both 43,000, and that of enzyme II was 95,000. All three enzymes required Mn2+ for maximal activity; Mg2+ could replace Mn2+ in the reaction of enzyme II, but only partially. In the presence of 0.1 M ammonium sulfate the activities of enzymes Ia and Ib were both completely inhibited, whereas enzyme II still showed 42% of its original activity. These findings suggest that there are two distinct types of poly(A) polymerase in Tetrahymena pyriformis.     

Characterization of supercoiled nucleoprotein complexes released from detergent-treated vaccinia virions.

Treatment of vaccinia virions with 1% sodium dodecyl sulfate in the absence of reducing agents resulted in the release of subviral particles termed "subnucleoids," which contained viral DNA in combination with four polypeptides with molecular weights of 90,000, 68,000, 58,000 and 10,000. Biochemical and electron microscopic studies showed that viral DNA in combination with these polypeptides was maintained in a superhelical configuration. When subnucleoids were "fixed" with glutaraldehyde and formaldehyde and then examined by electron microscopy, spherical particles were observed, in which the supercoiled DNA was folded into globular structures that were 20 to 60 nm in diameter and were interconnected by DNA-protein fibers resembling the nucleosome structures described for eucaryotic chromatin.     

Characterization of a protein kinase and two phosphate acceptor proteins from vaccinia virions.

The phosphorylation of two purified vaccinia virus proteins (Acceptors I and II) by a protein kinase isolated from vaccinia virus cores has been studied. Phosphorylation of viral acceptor proteins by the purified enzyme was dependent on the presence of ATP, Mg2+, and protamine or other basic proteins, and was maximal at alkaline pH values. Cyclic mononucleotides did not stimulate the vaccinia protein kinase under a variety of conditions. Protamine, however, was shown to function as an enzyme activator. In its presence, the purified vaccinia protein kinase phosphorylated mainly serine residues in Acceptor I, and predominantly threonine residues in Acceptor II. Phosphorylation of protamine accounted for less than 1% of the total 23P incorporation. Tryptic peptide maps prepared from 32P-labeled Acceptors I and II demonstrated that they contained different labeled peptide sequences and were, therefore, distinct protein species. From additional studies on both purified and virus-associated protein kinase it was concluded that various proteins affected the protein kinase reaction in one of three ways. One class of proteins served as phosphate acceptors, but only when another activator protein was present. A second class consisted of proteins that were strong activators but poor phosphate acceptors. The third class contained proteins that were fair phosphate acceptors, but which also activated the phosphorylation of other acceptor proteins.     

Solubilization of a protein synthesis inhibitor from vaccinia virions.

The protein synthesis inhibitor previously demonstrated to be associated with vaccinia cores was quantitatively solubilized from vaccinia virions or cores after an endogenous protein kinase reaction at pH 10. The presence of the inhibitor in the soluble extract correlated with the presence of soluble virion proteins phosphorylated in vitro. Partially purified inhibitor blocked methionyl-tRNAfMet-40S initiation complex formation, as was the case in cell-free extracts derived from vaccinia virus-infected cells.     

Polyamines in vaccinia virions and polypeptides released from viral cores by acid extraction.

Vaccinia virions propagated in the presence of [3H]ornithine were found to contain two labeled polyamines, spermine and spermidine. In complete virions the ratio of radioactively labeled spermine to spermidine was about 1:10, whereas in viral cores the ratio was 2:5. This suggests that some spermidine was preferentially lost during the conversion of virions to cores or that spermidine was present in the virions both inside and outside the core structure. Addition of [3H]ornithine to vaccinia virus-infected cells as late as 6 h postinfection demonstrated that, although the conversion of this precursor to polyamines was reduced by 50% or more as compared to mock-infected cells, complete inhibition of polyamine synthesis did not occur. Two percent or less of the total radioactivity associated with virions grown in the presence of [3H]ornithine was found to be acid soluble. Polyacrylamide gel electrophoretic analysis showed that all the structural polypeptides were labeled when virions were propagated in the presence of [3H]ornithine. When cores labeled with a mixture of 14C-labeled amino acids were extracted with 0.25 N H2SO4, 12 to 15% of the labeled core polypeptides were released and could be precipitated with acetone. About 40% of [3H]arginine-labeled polypeptides associated with cores were extracted with acid. Four polypeptides or groups of polypeptides were resolved after polyacrylamide gel electrophoresis analysis of the acid-soluble fraction of cores with molecular weights of about 58,000, 34,000, 24,000 and 10,000 to 12,000. About 40% of the [3H]arginine radioactivity extracted from cores coelectrophoresed with the 10,000 to 12,000-molecular weight polypeptide, indicating that this may represent an arginine-rich, histone-like structural polypeptide of the virion.     

Purification of mRNA guanylyltransferase and mRNA (guanine-7-) methyltransferase from vaccinia virions.

The sequences m7G(5')pppGm-and m7G(5')pppAm-are located at the 5' termini of vaccinia mRNAs. Two novel enzymatic activities have been purified from vaccinia virus cores which modify the 5' terminus of unmethylated mRNA. One activity transfers GMP from GTP to mRNA and is designated a GTP: mRNA guanylyltransferase. The second activity transfers a methyl group from S-adenosylmethionine to position 7 of the added guanosine and is designated a S-adenosylmethionine: mRNA (guanine-7-)methyltransferase. Advantage was taken of the selective binding of these activities to homopolyribonucleotides relative to DNA to achieve a 200-fold increase in specific activity. The guanylyl- and methyltransferase remained inseparable during chromatography on DNA-agarose, poly(U)-Sepharose, poly(A)-Sepharose, and Sephadex G-200 and during sedimentation through sucrose density gradients suggesting they were associated. A Stokes radius of 5.0 nm, an S20,w of 6.0 and a molecular weight of 127,000 were obtained by gel filtration on Sephadex G-200 and sedimentation in sucrose density gradients. Under denaturing conditions of sodium dodecyl sulfate-polyacrylamide gel electrophoresis two major polypeptides were detected in purified enzyme preparations. Their molecular weights of 95,000 and 31,400 suggested they were polypeptide components of the 127,000 molecular weight enzyme system.     

La Crosse virions contain a primer-stimulated RNA polymerase and a methylated cap-dependent endonuclease.

Uncapped reovirus mRNA extracted at late times from infected L-cells is preferentially translated in extracts from infected L-cells. However, translation of this uncapped, late, reovirus mRNA in extracts from infected cells is sensitive to inhibition by the cap analog m7GTP . These results imply that reovirus infection does not induce a transition from cap-dependent to cap-independent translation. Nevertheless, the results of in vitro translational competition experiments between L-cell mRNA and late viral mRNA were consistent with the view that reovirus does induce an alteration in the cap-dependent translational apparatus of L-cells. The reduced efficiency of translation of a variety of capped mRNAs in extracts from infected cells is also consistent with this notion. We further conclude that a factor exists in reovirus-infected L-cells that specifically stimulates translation of uncapped reovirus mRNAs.     

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).