Comparison of oligonucleotides produced by RNase T1 digestion of 7 S RNA from avian and murine oncornaviruses and from uninfected cells

Noncoding RNAs in Retrovirus Replication

Although a limited percentage of the genome produces proteins, approximately 90% is transcribed, indicating important roles for noncoding RNA (ncRNA). It is now known that these ncRNAs have a multitude of cellular functions ranging from the regulation of gene expression to roles as structural elements in ribonucleoprotein complexes. ncRNA is also represented at nearly every step of viral life cycles. This chapter will focus on ncRNAs of both host and viral origin and their roles in retroviral life cycles. Cellular ncRNA represents a significant portion of material packaged into retroviral virions and includes transfer RNAs, 7SL RNA, U RNA, and vault RNA. Initially thought to be random packaging events, these host RNAs are now proposed to contribute to viral assembly and infectivity. Within the cell, long ncRNA and endogenous retroviruses have been found to regulate aspects of the retroviral life cycle in diverse ways. Additionally, the HIV-1 transactivating response element RNA is thought to impact viral infection beyond the well-characterized role as a transcription activator. RNA interference, thought to be an early version of the innate immune response to viral infection, can still be observed in plants and invertebrates today. The ability of retroviral infection to manipulate the host RNAi pathway is described here. Finally, RNA-based therapies, including gene editing approaches, are being explored as antiretroviral treatments and are discussed.

Recruitment of 7SL RNA to assembling HIV-1 virus-like particles

Retroviruses incorporate specific host cell RNAs into virions. In particular, the host noncoding 7SL RNA is highly abundant in all examined retroviruses compared with its cellular levels or relative to common mRNAs such as actin. Using live cell imaging techniques, we have determined that the 7SL RNA does not arrive with the HIV-1 RNA genome. Instead, it is recruited contemporaneously with assembly of the protein HIV-1 Gag at the plasma membrane. Further, we demonstrate that complexes of 7SL RNA and Gag can be immunoprecipitated from both cytosolic and plasma membrane fractions. This indicates that 7SL RNAs likely interact with Gag prior to high-order Gag multimerization at the plasma membrane. Thus, the interactions between Gag and the host RNA 7SL occur independent of the interactions between Gag and the host endosomal sorting complex required for transport (ESCRT) proteins, which are recruited temporarily at late stages of assembly. The interactions of 7SL and Gag are also independent of interactions of Gag and the HIV-1 genome which are seen on the plasma membrane prior to assembly of Gag.

The Host RNAs in Retroviral Particles

As they assemble, retroviruses encapsidate both their genomic RNAs and several types of host RNA. Whereas limited amounts of messenger RNA (mRNA) are detectable within virion populations, the predominant classes of encapsidated host RNAs do not encode proteins, but instead include endogenous retroelements and several classes of non-coding RNA (ncRNA), some of which are packaged in significant molar excess to the viral genome. Surprisingly, although the most abundant host RNAs in retroviruses are also abundant in cells, unusual forms of these RNAs are packaged preferentially, suggesting that these RNAs are recruited early in their biogenesis: before associating with their cognate protein partners, and/or from transient or rare RNA populations. These RNAs' packaging determinants differ from the viral genome's, and several of the abundantly packaged host ncRNAs serve cells as the scaffolds of ribonucleoprotein particles. Because virion assembly is equally efficient whether or not genomic RNA is available, yet RNA appears critical to the structural integrity of retroviral particles, it seems possible that the selectively encapsidated host ncRNAs might play roles in assembly. Indeed, some host ncRNAs appear to act during replication, as some transfer RNA (tRNA) species may contribute to nuclear import of human immunodeficiency virus 1 (HIV-1) reverse transcription complexes, and other tRNA interactions with the viral Gag protein aid correct trafficking to plasma membrane assembly sites. However, despite high conservation of packaging for certain host RNAs, replication roles for most of these selectively encapsidated RNAs-if any-have remained elusive.

Analysis of the human immunodeficiency virus-1 RNA packageome

All retroviruses package cellular RNAs into virions. Studies of murine leukemia virus (MLV) revealed that the major host cell RNAs encapsidated by this simple retrovirus were LTR retrotransposons and noncoding RNAs (ncRNAs). Several classes of ncRNAs appeared to be packaged by MLV shortly after synthesis, as precursors to tRNAs, small nuclear RNAs, and small nucleolar RNAs were all enriched in virions. To determine the extent to which the human immunodeficiency virus (HIV-1) packages similar RNAs, we used high-throughput sequencing to characterize the RNAs within infectious HIV-1 virions produced in CEM-SS T lymphoblastoid cells. We report that the most abundant cellular RNAs in HIV-1 virions are 7SL RNA and transcripts from numerous divergent and truncated members of the long interspersed element (LINE) and short interspersed element (SINE) families of retrotransposons. We also detected precursors to several tRNAs and small nuclear RNAs as well as transcripts derived from the ribosomal DNA (rDNA) intergenic spacers. We show that packaging of a pre-tRNA requires the nuclear export receptor Exportin 5, indicating that HIV-1 recruits at least some newly made ncRNAs in the cytoplasm. Together, our work identifies the set of RNAs packaged by HIV-1 and reveals that early steps in HIV-1 assembly intersect with host cell ncRNA biogenesis pathways.

Host RNA Packaging by Retroviruses: A Newly Synthesized Story

A fascinating aspect of retroviruses is their tendency to nonrandomly incorporate host cell RNAs into virions. In addition to the specific tRNAs that prime reverse transcription, all examined retroviruses selectively package multiple host cell noncoding RNAs (ncRNAs). Many of these ncRNAs appear to be encapsidated shortly after synthesis, before assembling with their normal protein partners. Remarkably, although some packaged ncRNAs, such as pre-tRNAs and the spliceosomal U6 small nuclear RNA (snRNA), were believed to reside exclusively within mammalian nuclei, it was demonstrated recently that the model retrovirus murine leukemia virus (MLV) packages these ncRNAs from a novel pathway in which unneeded nascent ncRNAs are exported to the cytoplasm for degradation. The finding that retroviruses package forms of ncRNAs that are rare in cells suggests several hypotheses for how these RNAs could assist retrovirus assembly and infectivity. Moreover, recent experiments in several laboratories have identified additional ways in which cellular ncRNAs may contribute to the retrovirus life cycle. This review focuses on the ncRNAs that are packaged by retroviruses and the ways in which both encapsidated ncRNAs and other cellular ncRNAs may contribute to retrovirus replication.

Assembly properties of human immunodeficiency virus type 1 Gag-leucine zipper chimeras: implications for retrovirus assembly

Expression of the retroviral Gag protein leads to formation of virus-like particles in mammalian cells. In vitro and in vivo experiments show that nucleic acid is also required for particle assembly. However, several studies have demonstrated that chimeric proteins in which the nucleocapsid domain of Gag is replaced by a leucine zipper motif can also assemble efficiently in mammalian cells. We have now analyzed assembly by chimeric proteins in which nucleocapsid of human immunodeficiency virus type 1 (HIV-1) Gag is replaced by either a dimerizing or a trimerizing zipper. Both proteins assemble well in human 293T cells; the released particles lack detectable RNA. The proteins can coassemble into particles together with full-length, wild-type Gag. We purified these proteins from bacterial lysates. These recombinant "Gag-Zipper" proteins are oligomeric in solution and do not assemble unless cofactors are added; either nucleic acid or inositol phosphates (IPs) can promote particle assembly. When mixed with one equivalent of IPs (which do not support assembly of wild-type Gag), the "dimerizing" Gag-Zipper protein misassembles into very small particles, while the "trimerizing" protein assembles correctly. However, addition of both IPs and nucleic acid leads to correct assembly of all three proteins; the "dimerizing" Gag-Zipper protein also assembles correctly if inositol hexakisphosphate is supplemented with other polyanions. We suggest that correct assembly requires both oligomeric association at the C terminus of Gag and neutralization of positive charges near its N terminus.

Analysis of the contribution of cellular and viral RNA to the packaging of APOBEC3G into HIV-1 virions

Background

Efficient incorporation of the cellular cytidine deaminase APOBEC3G (APO3G) into HIV-1 virions is necessary for its antiviral activity. Even though cellular RNAs are known to be non-specifically incorporated into virus particles, we have previously found that encapsidation of APO3G into HIV-1 virions is specifically enhanced by viral genomic RNA. Intracellularly, APO3G was found to form large RNA-protein complexes involving a variety of cellular RNAs. The goal of this study was to investigate the possible contribution of host RNAs recently identified in intracellular APO3G ribonucleoprotein complexes to APO3G's encapsidation into HIV-1 virions.

Results

Our results show that 7SL RNA, a component of signal recognition particles, and hY1, hY3, hY4, hY5 RNAs were present in intracellular APO3G complexes and were packaged into HIV-1 particles lacking viral genomic RNA unlike APO3G, which was not packaged in significant amounts into genomic RNA-deficient particles. These results indicate that packaging of 7SL or hY RNAs is not sufficient for the packaging of APO3G into HIV-1 virions. We also tested the encapsidation of several other cellular RNAs including β-actin, GAPDH, α-tubulin, and small nuclear RNAs and determined their effect on the packaging of APO3G into nascent virions. Again, we were unable to observe any correlation between APO3G encapsidation and the packaging of any of these cellular RNAs.

Conclusion

The results from this study support our previous conclusion that viral genomic RNA is a critical determinant for APO3G incorporation into HIV-1 virions. While most cellular RNAs tested in this study were packaged into viruses or virus-like particles we failed to identify a correlation between APO3G encapsidation and the packaging of these cellular RNAs.

Selective and nonselective packaging of cellular RNAs in retrovirus particles

Assembly of retrovirus particles normally entails the selective encapsidation of viral genomic RNA. However, in the absence of packageable viral RNA, assembly is still efficient, and the released virus-like particles (termed "Psi-" particles) still contain roughly normal amounts of RNA. We have proposed that cellular mRNAs replace the genome in Psi- particles. We have now analyzed the mRNA content of Psi- and Psi+ murine leukemia virus (MLV) particles using both microarray analysis and real-time reverse transcription-PCR. The majority of mRNA species present in the virus-producing cells were also detected in Psi- particles. Remarkably, nearly all of them were packaged nonselectively; that is, their representation in the particles was simply proportional to their representation in the cells. However, a small number of low-abundance mRNAs were greatly enriched in the particles. In fact, one mRNA species was enriched to the same degree as Psi+ genomic RNA. Similar results were obtained with particles formed from the human immunodeficiency virus type 1 (HIV-1) Gag protein, and the same mRNAs were enriched in MLV and HIV-1 particles. The levels of individual cellular mRNAs were approximately 5- to 10-fold higher in Psi- than in Psi+ MLV particles, in agreement with the idea that they are replacing viral RNA in the former. In contrast, signal recognition particle RNA was present at the same level in Psi- and Psi+ particles; a minor fraction of this RNA was weakly associated with genomic RNA in Psi+ MLV particles.

7SL RNA, but not the 54-kd signal recognition particle protein, is an abundant component of both infectious HIV-1 and minimal virus-like particles

The virion incorporation of 7SL, the RNA component of the host signal recognition particle (SRP), has been shown for several simple retroviruses. Data here demonstrate that 7SL is also packaged by HIV-1, in sevenfold molar excess of genomic RNA. Viral determinants of HIV-1 genome and primer tRNA packaging were not required for 7SL incorporation, as virus-like particles with only minimal assembly components efficiently packaged 7SL. The majority of 7SL within cells resides in ribonucleoprotein complexes bound by SRP proteins, and most SRP protein exists in signal recognition particles. However, Western blot comparison of virion and cell samples revealed that there is at least 25-fold less SRP p54 protein per 7SL RNA in HIV-1 particles than in cells. Comparing 7SL:actin mRNA ratios in virions and cells revealed that 7SL RNA appears selectively enriched in virions.

Linkage of a 7S RNA sequence and kappa light chain genes in the mouse

A mouse 7S RNA cDNA plasmid clone was employed to identify and map DNA restriction fragment variants using recombinant inbred (RI) and congenic mouse strains. More than a dozen such restriction variants were identified and mapped to different regions of the mouse genome. One such variant, designated Rn7s-6, showed close linkage to the Ly-2,3-Igk-V (T lymphocyte antigens 2 and 3, kappa immunoglobulin variable region) cluster of markers on chromosome 6. No recombinants were detected among three of these markers in 59 RI strains. On the basis of these data, the Rn7s-6 sequence may be placed within 1.3 centimorgans of Ly-3 and one of the Igk-V-region markers, Igk-Ef1. Two mouse stocks with previously identified crossovers within the Ly-2,3-Igk-V region were used to sublocalize Rn7s-6. The results are consistent with the gene order (Ly-2, Ly-3)-(Rn7s-6, Igk-Ef1)-Igk-Ef2. Several mouse plasmacytomas, known to have various parts of the kappa chain complex deleted, retain the Rn7s-6 sequence. The Rn7s-6 variant is a plus/minus variant; no sequence allelic to Rn7s-6 is found in inbred strains that share the Ly-3a-Igk-Ef1a haplotype.

Properties of dispersed, highly repeated DNA within and near the hamster CAD gene

Dispersed, highly repeated DNA sequences were found within and near the Syrian hamster gene coding for the multifunctional protein CAD. Most of the repeated sequences were homologous to each other and had similar properties. They hybridized to many cytoplasmic polyadenylated RNAs and to 7S and 4.5S cytoplasmic non-polyadenylated RNAs. Cloned DNA fragments containing repeated sequences were transcribed in vitro by RNA polymerase III. The repeated sequences from Syrian hamsters share many properties with the Alu family of repetitive DNA from humans. The hamster sequences were homologous to total repetitive human DNA but only very weakly homologous to two cloned members of the human Alu family.

Signal recognition particle contains a 7S RNA essential for protein translocation across the endoplasmic reticulum

In addition to its previously characterized, six different polypeptide components, signal recognition protein--which functions in protein translocation across and integration into the endoplasmic reticulum membrane--contains a 7S RNA molecule. The RNA is closely identified with the small cytoplasmic 7SL RNA and is required for both structural and functional properties of signal recognition protein--which we therefore rename signal recognition particle.

Cloning and characterisation of the abundant cytoplasmic 7S RNA from mouse cells

A cDNA library has been prepared from mouse embryo small RNAs and screened for the presence of clones complementary to the highly abundant cytoplasmic 7S RNA. One clone (pA6) was selected which hybridized exclusively with 7S RNA on a Northern blot prepared from cytoplasmic RNA run on high resolution polyacrylamide/urea gels. Sequence analysis of this clone has shown that at least 65 nucleotides at the 5' end of 7S RNA are extensively homologous with the highly repeated mouse B1 family. Heterologous hybridisations between the cloned mouse 7S sequence and RNAs prepared from rat, human and chick cells have shown that the non-B1 part of the 7S RNA molecule has been highly conserved during recent eucaryotic evolution. There are multiple copies of 7S RNA genes in the genomes of mouse, human, rat and chick cells, but substantial differences exist in copy number and genomic organisation in these organisms.

The Alu family of dispersed repetitive sequences

A family of related sequences that includes approximately 500,000 members is the most prominent short dispersed repeat family in primate and rodent DNA's. The primate sequence is approximately 300 base pairs in length and is composed of two imperfectly repeated monomer units, whereas the rodent repeat consists of only a single monomer. Properties of this repeat sequence, its flanking sequences in chromosomal DNA, and RNA's transcribed from it suggest that it may be a mobile DNA element inserted at hundreds of thousands of different chromosomal locations.

Human 7SL RNA consists of a 140 nucleotide middle-repetitive sequence inserted in an alu sequence

We have cloned and sequenced a cDNA copy of in vitro-polyadenylated 7SL RNA of HeLa cells. The cloned fragment is 303 bp long and has a composite structure. A central block of 140 bp is homologous to a new set of human middle-repetitive sequences. This block appears to be inserted in an Alu consensus sequence, 100 bp from the 5' end and 40 bp from the 3' end of the Alu monomer. Two 6 bp direct repeats are found at the junction between the Alu flanking sequences and the central element. The analysis of several clones shows the existence of sequence microheterogeneity in the 5' portion of the molecule. The 7L DNA probably represents a subset of the Alu family of DNA, highly conserved in evolution.

Cloning and characterization of cDNA copies of the 7S RNAs of HeLa cells

We have cloned cDNA copies of in vitro adenylated 7S RNA of HeLa cells. The most representative clones in the library contain DNA fragments copied from the 7SL and 7SK small RNAs. The two classes of recombinants share no homology. The 7SL RNA contains at the 5' end of the molecule sequences homologous to the Alu sequence family. Hybridization to human genomic DNA shows that the 7SL and 7SK clones are homologous to two different families of repetitive sequences.

RNA-dependent RNA polymerase activity in coronavirus- infected cells

An enzymatic activity which incorporates [3H]UMP into acid-precipitable material in the presence of endogenous template was found in the cytoplasm of porcine cells infected with the transmissible gastroenteritis virus of swine. This activity was not found in uninfected control cells, nor was it found in purified virus. The activity was associated with the mitochondrial fraction of infected cells, suggesting that the enzyme is membrane bound. The activity required the presence of all three ribonucleoside triphosphates in addition to [3H]UTP, and it was not inhibited by actinomycin D. The heated product was digested by RNase but not by DNase. Mg2+ was required for enzymatic activity, and its optimal concentration was approximately 5 mM. The size of the in vitro products was compared by electrophoresis with that of in vivo-synthesized virus-specified RNA to confirm the viral specificity of the polymerase activity. Virus-specified RNA from infected cells consisted of 10 species of single-stranded, polyadenylated RNA with molecular weights of 6.8 X 10(6), 6.2 X 10(6), 3.15 X 10(6), 1.40 X 10(6), 1.05 X 10(6), 0.94 X 10(6), 0.66 X 10(6), 0.39 X 10(6), 0.34 X 10(6), and 0.24 X 10(6). In vitro synthesized RNA consisted of a high-molecular-weight species, of apparently higher molecular weight than genomic RNA, and two single-stranded species that electrophoretically comigrated with the species of 1.40 X 10(6) and 0.66 X 10(6) molecular weight made in vivo.

Properties of dispersed, highly repeated DNA within and near the hamster CAD gene

Dispersed, highly repeated DNA sequences were found within and near the Syrian hamster gene coding for the multifunctional protein CAD. Most of the repeated sequences were homologous to each other and had similar properties. They hybridized to many cytoplasmic polyadenylated RNAs and to 7S and 4.5S cytoplasmic non-polyadenylated RNAs. Cloned DNA fragments containing repeated sequences were transcribed in vitro by RNA polymerase III. The repeated sequences from Syrian hamsters share many properties with the Alu family of repetitive DNA from humans. The hamster sequences were homologous to total repetitive human DNA but only very weakly homologous to two cloned members of the human Alu family.

Transcriptional analysis of interspersed repetitive polymerase III transcription units in human DNA

The template for RNA polymerase III in vitro transcription found on the human DNA clone pJP53 was shown in the previous paper to enclose a member of the Alu famiy of interspersed repetitive DNA sequences. We have mapped this transcript onto its template in greater detail by comparison of the template DNA sequence to the base composition of the Tl ribonuclease digestion products of the in vitro transcript. We find that the 5' end of the transcript lies in close proximity to the 5' end of the conserved Alu family sequence as analyzed in the preceding paper. The 3' end of the transcript appears to terminate in a U-rich region beyond the region of Alu family sequence conservation. Analysis of cellular RNA by Northern blotting and hybridization with a DNA probe derived from another Alu family transcription template demonstrates abundant representation of sequences homologous to the reiterated DNA. Cytoplasmic, nonpolyadenylated RNA from human and murine cells contains a monodisperse, 300 nucleotide species, recently determined by Weiner (4) to be the 7S RNA. In contrast, the Alu-homologous transcripts are heterodisperse in mRNA and hnRNA, with the highest specific representation of Alu family sequences being found in oligo(dT)-retained hnRNA.

Subcellular localization of low-molecular-weight RNA components in rat liver

The subcellular localization of the four major low-molecular-weight RNA components, D, C, A and L, was studied in rat liver cells. The cells were fractionated by a non-aqueous technique into a nuclear and a cytoplasmic fraction. The cytoplasm contained 43% of component D, 57% of component C and more than 80% of component L.

An abundant cytoplasmic 7S RNA is complementary to the dominant interspersed middle repetitive DNA sequence family in the human genome

Evidence is presented that a homogeneous cytoplasmic species known as 7S RNA is the only abundant RNA in uninfected HeLa cells which can form strong hybrids with the dominant family of middle repetitive DNA sequences in the human genome. These DNA sequences are known collectively as the Alu family, because most of them share a common Alu I restriction site. When purified 7S RNA was hybridized to three different genomic clones containing Alu family DNA sequences, a specific region (or regions) comprising at most half the RNA sequence was protected from mild digestion with T1 ribonuclease; moreover, the hybrids between 7S RNA and cloned Alu family DNA wer imperfect, since T1 RNAase was able to nick the protected 7S RNA sequences under conditions where a true RNA: DNA duplex would have been resistant. This suggests that 7S RNA is encoded either by a small subset of the 300,000 Alu family sequences in the human genome or by an entirely different family of genes. The sequence of 7S RNA has been highly conserved through recent evolution, and in both avian and murine cells the RNA is selectively incorporated into oncornavirus particles during productive infection. The cellular function of 7S RNA is unknown.

Genome of porcine transmissible gastroenteritis virus

The Purdue strain of transmissible gastroenteritis virus, a porcine coronavirus, was grown to titers of greater than 10(8) PFU/ml in a swine testicle cell line, and the RNA was isotopically labeled with [3H]uridine. The RNA was extracted from purified virus and was found to have the following properties. (i) It consisted primarily of a homogeneous large-molecular-weight species which electrophoretically migrated with an apparent molecular weight of 6.8 X 10(6) under denaturing conditions. (ii) It migrated electrophoretically at the same rate on nondenaturing gels before and after heat denaturation, suggesting that it does not consist of subunits. (iii) It was susceptible to pancreatic RNase A digestion in high (0.3 M) NaCl. (iv) It was polyadenylated to the extent that greater than 60% of the native RNA bound to oligodeoxythymidilic acid-cellulose under conditions of high (0.5 M) NaCl. RNA extracted from virions was infectious. This coronavirus can therefore be characterized as a positive-strand RNA virus.

Bovine coronavirus genome

The tissue culture-adapted strain (Mebus) of the bovine coronavirus was grown to titers of greater than 10(7) 50% tissue culture infective doses per ml in secondary bovine embryo kidney cells, and the RNA was isotopically labeled with [3H]uridine. The RNA was extracted from purified virus and was found to have the following properties. (i) It consisted primarily of a homogeneous large-molecular-weight species which comigrated electrophoretically with vesicular stomatitis viral RNA and therefore had an apparent molecular weight of 3.8 X 10(6). (ii) It remained as a 3.8 x 10(6)-molecular-weight molecule after heat denaturation when rapidly harvested virus was examined. (iii) It was 80% susceptible to pancreatic RNase A digestion in high (0.3 M) NaCl, and the 20% resistant fraction was 4S to 7S in size. (iv) It was polyadenylated to the extent that 40 and 60% of the native RNA bound to polyuridylic acid-Sepharose and oligodeoxythymidylic acid-cellulose, respectively, under conditions of high (0.5 M) NaCl.

Some properties of the small homodisperse RNAs in the cytoplasm of HeLa cells

Two homodisperse, methylated, small molecular weight RNAs, that have been previously described in HeLa cells and that are apparently coded by the nuclear genome, appear in the cytoplasm shortly after transcription and are detected in the cytoplasm with a half-life of a few minutes. Under the denaturing conditions of urea-polyacrylamide gel electrophoresis, the migration of these RNAs indicated nucleotide lengths around 190 (A) and 160 (B). Neither A nor B RNAs differed in their electrophoretic mobility when comparing samples from 10 to 120 min incubation with [3H]uridine. When the detergent-treated postmitochondrial fraction from cells incubated for 60 min with [3H]-uridine was centrifuged through a 0.3 M NaC1/0.5 M sucrose cushion, the resulting pellet showed a marked increase in the ratio of counts A/4 S RNA, and particularly B/4 S RNA, suggesting that these RNAs might not exist free in the cytoplasm. At low concentrations of camptothecin (1-2.5 mug/ml), the accumulation of A and B RNA was greatly suppressed, while the synthesis of 4 S and 5 S RNA was less affected. Their accumulation (particularly that of B RNA) was enhanced when cells were exposed to inhibitors of various steps of protein synthesis (puromycin, cycloheximide, pactamycin, emetine). Pulse-chase experiments with actinomycin D during suppression of protein synthesis indicated that, at least in the case of B RNA, this increased accumulation was due, at least partly, to a marked prolongation of its half-life.

Synthesis of viral proteins by the avian myeloblastosis viral core component

Avian Myeloblastosis Viral (AMV) core component was isolated and shown to synthesize AMV proteins in vitro. This reaction was linearly dependent on viral core concentration, proceeded linearly with time, and was inhibited by puromycin and aurintricarboxylic acid. The proteins synthesized in vitro co-electrophoresed and co-chromatographed with known proteins, and were immunoprecipitated by total and monospecific antibodies to known AMV proteins.

The 7S RNA common to oncornaviruses and normal cells is associated with polyribosomes

The 7S RNA species first demonstrated in avian and murine oncornaviruses and later in normal, uninfected cells is found associated in part with cellular polyribosomes. A molar ratio of 7S RNA to 5S ribosomal RNA of 0.05 indicates that there is approximately one mole of 7S RNA per mole of messenger RNA. Dissociation of polyribosomes with dimethylsulfoxide results in a marked decrease in the sedimentation rate of the 7S RNA. The dimethylsulfoxide-induced dissociation of polyribosomes and the concomitant movement of the 7S RNA from the polyribosome region into lighter regions of a sucrose gradient are both inhibited by cycloheximide, indicating that the 7S RNA is indeed associated with polyribosomes and not with a ribonucleoprotein particle sedimenting with polyribosomes.

Identity of the two 8S RNA components of the mouse sarcoma virus (Moloney)

The two 8S A and B RNAs of the Mouse Sarcoma Virus (Moloney) can be converted by heating into a homogeneous population. After digestion with T(1) RNAse, they give identical fingerprints. It is concluded that these two molecules represent conformational isomers of the same molecular species.

Virion-associated RNA primer for Rous sarcoma virus DNA synthesis: isolation from uninfected cells

Uninfected chicken, duck, rat, and human fibroblast cells in culture contained a tRNA-like RNA molecule which was structurally identical to a virion-associated RNA primer for in vitro Rous sarcoma virus DNA synthesis. This primer RNA appeared to be a normal tRNA of these cells. It was not found in a number of lower eukaryotic cells or in Escherichia coli.