Identification of terminal adenylyl transferase activity of the poliovirus polymerase 3Dpol

Comparison of the replication of positive-stranded RNA viruses of plants and animals

It is clear from the experimental data that there are some similarities in RNA replication for all eukaryotic positive-stranded RNA viruses—that is, the mechanism of polymerization of the nucleotides is probably similar for all. It is noteworthy that all mechanisms appear to utilize host membranes as a site of replication. Membranes appear to function not only as a way of compartmentalizing virus RNA replication but also appear to have a central role in the organization and functioning of the replication complex, and further studies in this area are needed. Within virus supergroups, similarities are evident between animal and plant viruses—for example, in the nature and arrangements of replication genes and in sequence similarities of functional domains. However, it is also clear that there has been considerable divergence, even within supergroups. For example, the animal alpha-viruses have evolved to encode proteinases which play a central controlling function in the replication cycle, whereas this is not common in the plant alpha-like viruses and even when it occurs, as in the tymoviruses, the strategies that have evolved appear to be significantly different. Some of the divergence could be host-dependent and the increasing interest in the role of host proteins in replication should be fruitful in revealing how different systems have evolved. Finally, there are virus supergroups that appear to have no close relatives between animals and plants, such as the animal coronavirus-like supergroup and the plant carmo-like supergroup.

In vivo restoration of biologically active 3' ends of virus-associated RNAs by nonhomologous RNA recombination and replacement of a terminal motif

Sequences at the 3' ends of plus-strand RNA viruses and their associated subviral RNAs are important cis elements for the synthesis of minus strands in vivo and in vitro. All RNAs associated with turnip crinkle virus (TCV), including the genomic RNA (4,054 bases) and satellite RNAs (sat-RNAs) such as sat-RNA D (194 bases), terminate with the motif CCUGCCC. While investigating the ability of in vivo-generated recombinants between sat-RNA D and TCV to be amplified in plants, we discovered that sat-RNA D, although truncated by as many as 15 bases in the chimeric molecules, was released from the chimeric transcripts and amplified to high levels. The "new" sat-RNA D molecules nearly all terminated with the motif (C1-2)UG(C1-3) (which may begin with 1 or 2 cytosines and end with 1, 2, or 3 cytosines), which was similar or identical to the natural sat-RNA D 3' end. The new sat-RNA D also contained between 1 and 22 bases of heterogeneous sequence upstream from the terminal motif, which, in some cases, was apparently derived from internal regions of either the plus or minus strand of the TCV genomic RNA. Since most of these internal genomic RNA sequences within TCV were not adjacent to (C1-2)UG(C1-3), at least two steps were required to produce new sat-RNA D 3' ends: nonhomologous recombination with the TCV genomic RNA followed by the addition or modification of the terminus to generate the (C1-2)UG(C1-3) motif.

Mutational analysis of the 3'-terminal extra-cistronic region of poliovirus RNA: secondary structure is not the only requirement for minus strand RNA replication

A series of mutations were introduced in the 3'-terminal untranslated region (3'-UTR) of full-length infectious poliovirus cDNA clones, and following transfection of COS-1 cells the ability of these constructs to generate viable viral particles and/or to support viral RNA synthesis was assayed. Substitution of the 3'-UTR of poliovirus RNA with the equivalent sequences of HAV RNA abrogated viral RNA replication, whereas the introduction of extended 'foreign' sequences between the open reading frame and the 3'-UTR was well tolerated. Point mutation that either destabilized the stem-and-loop structure or altered the sequence of the loop in domain 'Y' (nomenclature as per Pilipenko et al., [Nuclei Acids Res. 20 (1992) 1739-1745]) abolished both the infectivity and viral RNA synthesis. These were not restored by compensatory mutation that reconstructed the native secondary structure of this domain, suggesting that the secondary/tertiary folding of the 3'-UTR is not the only determinant for template recognition at initiation of RNA synthesis, but rather that a specific primary sequence is indeed required.

Poliovirus protein 3AB forms a complex with and stimulates the activity of the viral RNA polymerase, 3Dpol

Poliovirus protein 3B (also known as VPg) is covalently linked to the 5' ends of both genomic and antigenomic viral RNA. Genetic and biochemical studies have implicated protein 3AB, the membrane-bound precursor to VPg, in the initiation of genomic RNA synthesis. We have purified 3AB to near homogeneity following thrombin cleavage of purified glutathione S-transferase-3AB. When added to transcription reaction mixtures catalyzed by poliovirus RNA polymerase (3Dpol), 3AB stimulated RNA synthesis up to 75-fold with oligo(U)-primed virion RNA, globin mRNA, and unprimed synthetic, full-length minus-strand viral RNA as the templates. Synthetic VPg also stimulated RNA synthesis but was only 1 to 2% as effective as 3AB on a molar basis. The increased level of transcription was not the result of enhancing the elongation rate of the polymerase. No evidence was found for uridylylation of 3AB or for covalent linkage to RNA transcription products. 3AB sedimented as a multimer in glycerol gradients. In the presence of the polymerase, the sedimentation rate of both proteins increased, suggesting the formation of a complex. Detergent prevented both multimerization and complex formation. The polymerase also bound to immobilized glutathione S-transferase-3AB; this procedure was used to purify the polymerase to near homogeneity. These results suggest a mechanism for bringing together 3AB, 3Dpol (or its precursor 3CD), and viral RNA in host cell membranous vesicles in which all viral RNA synthesis occurs.

Inefficient complementation activity of poliovirus 2C and 3D proteins for rescue of lethal mutations

Poliovirus (PV) 2C protein is a nonstructural polypeptide involved in viral RNA replication, whose biochemical activity(ies) in this process has not been defined. By using site-directed mutagenesis, it was shown previously that disruption of nucleotide-binding motifs present in this protein abolished viral RNA synthesis (C. Mirzayan and E. Wimmer, Virology 189:547-555, 1992; N. L. Teterina, K. M. Kean, E. Gorbalenya, V. I. Agol, and M. Girard, J. Gen. Virol. 73:1977-1986, 1992). We have tested whether PV 2C or 2BC protein provided in trans could rescue the replication of these mutated genomes. Rescuing proteins were provided either by cotransfection with helper chimeric PV-coxsackievirus genomes or by expression in cells with a vaccinia virus-T7 RNA polymerase transient-expression system. We report here that replication of mutated RNAs genomes was poorly supported in trans both by helper genomes and by expressed 2C or 2BC proteins. Similarly, very inefficient complementation was observed for two mutated genomes with lethal lesions in 3D polymerase coding sequence. Our results indicate that poliovirus RNA replication shows marked preference for proteins contributed in cis.