Poly(A) polymerase activity in reovirus

mRNA poly(A) tail, a 3' enhancer of translational initiation

To evaluate the hypothesis that the 3' poly(A) tract of mRNA plays a role in translational initiation, we constructed derivatives of pSP65 which direct the in vitro synthesis of mRNAs with different poly(A) tail lengths and compared, in reticulocyte extracts, the relative efficiencies with which such mRNAs were translated, degraded, recruited into polysomes, and assembled into messenger ribonucleoproteins or intermediates in the translational initiation pathway. Relative to mRNAs which were polyadenylated, we found that nonpolyadenylated [poly(A)-]mRNAs had a reduced translational capacity which was not due to an increase in their decay rates, but was attributable to a reduction in their efficiency of recruitment into polysomes. The defect in poly(A)- mRNAs affected a late step in translational initiation, was distinct from the phenotype associated with cap-deficient mRNAs, and resulted in a reduced ability to form 80S initiation complexes. Moreover, poly(A) added in trans inhibited translation from capped polyadenylated mRNAs but stimulated translation from capped poly(A)- mRNAs. We suggest that the presence of a 3' poly(A) tail may facilitate the binding of an initiation factor or ribosomal subunit at the mRNA 5' end.

mRNA poly(A) tail, a 3' enhancer of translational initiation

To evaluate the hypothesis that the 3' poly(A) tract of mRNA plays a role in translational initiation, we constructed derivatives of pSP65 which direct the in vitro synthesis of mRNAs with different poly(A) tail lengths and compared, in reticulocyte extracts, the relative efficiencies with which such mRNAs were translated, degraded, recruited into polysomes, and assembled into messenger ribonucleoproteins or intermediates in the translational initiation pathway. Relative to mRNAs which were polyadenylated, we found that nonpolyadenylated [poly(A)-]mRNAs had a reduced translational capacity which was not due to an increase in their decay rates, but was attributable to a reduction in their efficiency of recruitment into polysomes. The defect in poly(A)- mRNAs affected a late step in translational initiation, was distinct from the phenotype associated with cap-deficient mRNAs, and resulted in a reduced ability to form 80S initiation complexes. Moreover, poly(A) added in trans inhibited translation from capped polyadenylated mRNAs but stimulated translation from capped poly(A)- mRNAs. We suggest that the presence of a 3' poly(A) tail may facilitate the binding of an initiation factor or ribosomal subunit at the mRNA 5' end.

Activation and characterization of the reovirus transcriptase: genetic analysis

We studied the ability of chymotrypsin to activate the transcriptases of the three serotypes of reovirus. When we used conditions that reproducibly caused the activation of type 3 transcriptase by chymotrypsin alone, type 2 transcriptase was sometimes activated, and type 1 transcriptase was never activated. Using intertypic recombinants containing various combinations of genome segments from reovirus types 3 and 1, we showed that the M2 segment determined this difference. Biochemical experiments indicated that the digestion of reovirus type 1 by chromotrypsin was blocked at an intermediate stage in uncoating. We found conditions which reproducibly activated the transcriptases of all three serotypes. This allowed us to compare the biochemical properties of the three transcriptases. Although the monovalent cation preferences, divalent cation preferences and optima, and temperature optima of type 1, 2, and 3 transcriptases were indistinguishable, the pH activity curves were reproducibly different. The largest difference was between type 2 and 3 transcriptases; the pH optimum of type 2 transcriptase was lower than the pH optimum of type 3 transcriptase. Using intertypic recombinants containing various combinations of genome segments from reovirus types 2 and 3, we demonstrated that the L1 segment specified this difference.

Virion DNA-independent RNA polymerase from Saccharomyces cerevisiae

The "killer" plasmid and a larger double-stranded RNA plasmid of yeast exist in intracellular virion particles. Purification of these particles from a diploid killer strain of yeast (grown into stationary growth on ethanol) resulted in co-purification of a DNA-independent RNA polymerase activity. This activity incorporates and requires all four ribonucleoside triphosphates and will not act on deoxyribonucleoside triphosphates. The reaction requires magnesium, is inhibited by sulfhydryl-oxidizing reagents and high concentrations of monovalent cation, but is insensitive to DNase, alpha-amanitin, and actinomycin D. Pyrophosphate inhibits the reaction as does ethidium bromide. Exogenous nucleic acids have no effect on the reaction. The product is mostly single-stranded RNA, some of which is released from the enzymatically active virions.

mRNA capping enzymes are masked in reovirus progeny subviral particles

We examined the enzyme activities associated with progeny subviral particles isolated from L-cells infected with reovirus at 12 h postinfection. Activities normally present in reovirus cores were also found to be present in the progeny subviral particles, with the exception of the capping enzymes. The methylase and guanyl transferase activities, which constitute the capping system, were present in a masked form that could be activated by chymotrypsin digestion. The appearance of these progeny subviral particles in infected cells coincided with the time when mRNA synthesis was maximal, suggesting that viral mRNA synthesized at later times is uncapped.

Reovirus-specific enzyme(s) associated with subviral particles responds in vitro to polyribocytidylate to yield double-stranded polyribocytidylate-polyriboguanylate

In reovirus-infected cells, virus-specific particles accumulate that have associated with them a polyribocytidylate [poly(C)]-dependent polymerase. This enzyme copies in vitro poly(C) to yield the double-stranded poly(C).polyriboguanylate [poly(G)]. The particles with poly(C)-dependent polymerase were heterogeneous in size, with most sedimenting from 300S to 550S. Exponential increase in these particles began at 23 h, and maximal amounts were present by 31 h, the time of onset of exponential growth of virus at 30 degrees C. Maximal amounts of particles with active transcriptase and replicase were present at 15 and 18 h after infection. Thereafter, there was a marked decrease in particles with active transcriptase and replicase until base line levels were reached at 31 h. Thus, the increase in poly(C)-responding particles occurred coincident with the decrease in particles with active transcriptase and replicase. The requirement for poly(C) as template was specific because no RNA was synthesized in vitro in response to any other homopolymer, including 2'-O-methyl-poly(C). Synthesis was optimal in the presence of Mn(2+) as the divalent cation, and no primer was necessary for synthesis. In contrast, the dinucleotide GpG markedly stimulated synthesis in the presence of 8 mM Mg(2+). The size of the poly(C).poly(G) synthesized in vitro was dependent on the size of the poly(C) used as template. This suggested that the whole template was copied into a complementary strand of similar size. The T(m) of the product was between 100 and 130 degrees C. Hydrolysis of the product labeled in [(32)P]GMP with alkali or RNase T2 yielded GMP as the only labeled mononucleotide. This does indicate that the synthesis of the poly(G) strand in vitro did not proceed by end addition to the poly(C) template, but proceeded on a separate strand.

A kinetic and structural characterization of adenosine-5'-triphosphate: ribonucleic acid adenylyltransferase from Pseudomonas putida

A catalytic and structural study of ATP:RNA adenylyltransferase (EC 2.7.7.19) from the particulate fraction of Pseudomonas putida was made. During the large-scale purification of this enzyme, designated adenylyltransferase B, a previously undetected ATP-incorporating activity, designated adenylyltransferase A, was observed. Adenylyltransferases A and B were indistinguishable catalytically; however, they differed in their chromatographic and sedimentation properties. Adenylyltransferases A and B were resolved by phosphocellulose, by poly (U)-Sepharose and by Bio-Gel P-100 chromatographies. Adenylytransferase A was determined to have a sedimentation coefficient (S020,w) of 9.3 S and B of 4.3 S. The molecular weight of adenylyltransferase A was estimated to be 185000 and that of adenylyltransferase B to be 50000-60000. Apparently, adenylyltransferase A was generated from adenylyltransferase B during the purification. The AMP incorporation catalyzed by adenylyltransferases A and B was inhibited by two derivatives of the antibiotic rifamycin, AF/013 (50% at 5 mug/ml) and AF/DNFI (50% at 10 mug/ml). The 5'-triphosphate derivative (3'-dATP) of the drug cordycepin (3'-deoxyadenosine/ was a competitive inhibitor with ATP for both adenylyltransferases. The Ki for 3'-deoxyadenosine 5'-triphosphate was 6 - 10(-4)--10 - 10(-4) M, while the Km for ATP was 1 - 10(-4)--2 - 10(-4) M. Several other anaolgs of ATP, 2'-deoxyadenosine 5' triphosphate, 2'-O-methyl ATP, or the fluorescent 3-beta-D-ribofuranosylimidazo [2,1-i] purien 5'-triphosphate did not affect the activity of adenylyltransferase A or B. Poly(U) and poly(dT) were competitive inhibitors of the ribosomal RNA-primed polymerization reaction. The Ki for poly(U) or poly(dT), in terms of nucleotide phosphate, was 4 - 10-6)--10 - 10(-6) M for adenylyltransferases A and B, compared to 2 - 10(-4)--4 - 10(-4) M for the Km of ribosomal RNA. The inhibition was a result of the competition between the non-priming poly(U), or poly(dT), and ribosomal RNA for the primer binding site on the enzyme.

In vitro methylation of nascent reovirus mRNA by a virion-associated methyl transferase

Chymotrypsin-derived cores, but not virions, catalyze the transfer of methyl groups from S-adenosyl methionine (SAM) to nascent mRNA synthesized in vitro by the core polymerase. The reaction requires Mg(++), and is dependent on the presence of all 4 ribonucleoside triphosphates (rNTPs). Methylation proceeds optimally at 51 degrees C. All ten species of mRNA become methylated during transcription and it is estimated that one methyl residue is incorporated per RNA chain. Experiments designed to determine the location of the methylated nucleotide clearly demonstrate that methylation occurs exclusively at the 5' ends of nascent mRNA.

Origin of reovirus oligo(A)

Reovirus contains about 1,200 molecules per virion of oligo(A) of chain length 10 to 15 nucleotides in addition to the 10 double-stranded genome segments. Virions purified from infected BHK, HeLa, or L cells had similar amounts of oligo(A) of the same composition, indicating that it is a virus-specific product. Although conversion of virions to cores by chymotryptic digestion resulted in an almost complete loss of oligo(A) and a marked decrease in infectivity, the infectivity could be partially restored by adsorbing cores to cells in the presence of Kaopectate. Core-infected cells yielded virions that contained a normal complement of oligo(A). The results indicate that oligo(A) is not essential for infectivity or required as a primer/template for its own synthesis.