The world of animal viruses appears to offer
an unfathomable diversity of specimens but, as
the molecular biology of the replication of many
viruses has been studied, a pattern of behavior
has emerged. The viruses can be divided into
classes, each of which has its own method of
transmitting its genetic information from one
generation to the next and its own style of expressing
its genetic information. Although in
some cases the data are still fragmentary it is
possible to outline the behavior of these systems
and to place them in a formal scheme. In this
paper I will present such a scheme and I will
discuss in some detail the behavior of three viral
systems which have been investigated in my
laboratory. Furthermore, I will discuss some of
the implications of the existence of these viral
systems in the context of the behavior of normal
cells.

Expression of Animal Virus Genomes

ADP-Ribosylation factor is a subunit of the coat of Golgi-derived COP-coated vesicles: A novel role for a GTP-binding protein

Domains have been identified in the Gag proteins of a number of retroviruses, and in the matrix proteins of the rhabdoviruses and filoviruses, that play a critical role in the pinching off of virus particles from the plasma membrane. These sequences are collectively termed late or L domains to reflect their function late in the virus budding process. One of the intriguing features of these L domains is that they all contain highly conserved motifs known to mediate protein-protein interactions between cellular proteins. Three classes of motifs have been defined in viral L domains: PTAP, PPXY, and YXXL. In each case, the integrity of these motifs appears to be essential for L domain activity, suggesting that L domains function by interacting with a host factor(s). This minireview summarizes the present state of our knowledge concerning viral L domain function and describes recent tantalizing clues regarding the identity of the cellular partners with which L domains interact.

Viral Late Domains

Publisher Summary The discovery of RNA-directed DNA synthesis in disrupted virions of RNA tumor viruses added strong support to the hypothesis that information transfer from RNA to DNA exists in biological system. The chapter discusses the properties of the endogenous reaction carried out by the virion DNA polymerase. To study the endogenous reaction disrupted, virions are incubated with substrates in the absence of any added template and synthesis of DNA is observed using the RNA present in the virions as template. The chapter also discusses the general implications of RNA-directed DNA synthesis in relation to tumor viruses, neoplastic cells, and normal cells. It is hypothesized that the leukovirus RNA-directed DNA polymerase activity is an integral part of the ribonucleoprotein core of the virions. The cores are synthesized in cells as precursor particles and then are incorporated into complete virions when the virions are assembled by budding at the cell surface. This core enzyme system contains not only the template-primer RNA, a DNA polymerase that can transfer information from RNA to double-stranded DNA, but ancillary enzymes, such as polynucleotide ligase and nucleases, which may aid in integrating the viral information with cellular DNA. This suggests that the virion enzyme activity is related to normal cellular DNA polymerases, and that there are homologies between the amino acid sequences of the viral enzyme and normal cellular enzymes. The relationship of RNA-directed DNA synthesis to neoplasia depends upon the relationship of RNA tumor viruses to neoplasia, which is supported by three general hypotheses: the provirus model, the oncogene model, and the protovirus model.

RNA-Directed DNA Synthesis and RNA Tumor Viruses

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.

Defective interfering (DI) particles were discovered three decades ago by von Magnus (1947) using the influenza virus system. He called them “incomplete” or “immature” particles. Even though they could not be isolated or characterized biochemically, a great deal was learned about them from their physiological interactions with the host and from their interference with the multiplication of “infectious” standard virus (Gard and von Magnus, 1947; Bernkopf, 1950; von Magnus, 1951). Reviews by Henle (1950), by von Magnus (1954), and by Schlesinger (1959) on viral interference discuss the earlier work on this particular homologous interference caused by incomplete virus.

Defective Interfering Animal Viruses

The matrix (M) protein of vesicular stomatitis virus regulates transcription

Formation of pseudotypes between murine RNA tumor viruses and vesicular stomatitis virus (VSV) has been confirmed Pseudotypes of VSV genomes coated by the surface envelope from an N-tropic tumor virus grew equally well in cells homozygous for either the Fv-1n or Fv-1b alleles Therefore, the product of the Fv-1 locus, which restricts growth of murine RNA tumor viruses, must act on an intracellular aspect of tumor virus replication, a step after attachment and penetration

Growth of Pseudotypes of Vesicular Stomatitis Virus with N-Tropic Murine Leukemia Virus Coats in Cells Resistant to N-Tropic Viruses

The virions of Newcastle disease virus (NDV) contained an enzyme that catalyzed the incorporation of ribonucleotides into ribonucleic acid (RNA). Optimal conditions for this polymerase activity were identical to the conditions for the vesicular stomatitis virus (VSV) polymerase, and both enzymes were active for longer times at 32 C than at 37 C. However, the specific activity of the NDV polymerase was less than 3% that of the VSV polymerase. Product RNA species from the NDV and VSV polymerase reactions annealed specifically to the homologous virion RNA species. Transcriptive intermediates containing product RNA attached to the respective virion RNA could be identified in both systems.

Ribonucleic Acud Polymerase in Virions of Newcastle Disease Virus: Comparison with the Vesicular Stomatitis Virus Polymerase

The temperature-sensitive mutant ts114 of vesicular stomatitis virus is temperature-sensitive in both primary and secondary transcription, but not in the replication of 40S RNA. The synthesis of 40S RNA is specifically inhibited when protein synthesis is shut off. The addition of cycloheximide to cells infected by ts114, rapidly inhibits RNA replication at the permissive and nonpermissive temperatures. However, the addition of cycloheximide at the nonpermissive temperature also results in almost complete recovery of transcription to the level found at the permissive temperature. Other inhibitors of protein synthesis applied at the nonpermissive temperature result in the same recoverability of the apparent temperature-sensitive lesion. Ribonucleic acid products synthesized by ts114 at the nonpermissive temperature in the presence of cycloheximide are identical to virus-specific messenger RNA by the criteria of size and complementarity to virion RNA. When mutant-infected cells are shifted to the nonpermissive temperature, incubated for a period of time, and then treated with cycloheximide, the ratio of transcriptive to replicative activity varies depending on the time at which radioactive precursor is added to cells. This suggests an interdependence between replication and transcription during virus-specific RNA synthesis.

Alice S. Huang

Papers

Defective Interfering Animal Viruses

The matrix (M) protein of vesicular stomatitis virus regulates transcription

Growth of Pseudotypes of Vesicular Stomatitis Virus with N-Tropic Murine Leukemia Virus Coats in Cells Resistant to N-Tropic Viruses

Ribonucleic Acud Polymerase in Virions of Newcastle Disease Virus: Comparison with the Vesicular Stomatitis Virus Polymerase

RNA Synthesis of Vesicular Stomatitis Virus V. INTERACTIONS BETWEEN TRANSCRIPTION AND REPLICATION