5'-Terminal sequences and coding region of late simian virus 40 mRNAs are derived from noncontiguous segments of the viral genome

The region of the simian virus 40 genome complementary to the 5' end of the most abundant poly(A)-containing 19S and 16S mRNAs was mapped by hybridization of double-labeled RNA ([3H]methyl group and [14C]uridine) to specific DNA fragments. Chemical identification of methylated residues indicated that a common "leader" sequence adjacent to the 5' terminus of both 19S and 16S mRNA is transcribed from DNA sequences located between 0.67 and 0.76 map units. The estimated size of this "leader" RNA, which does not code for any known viral protein, is 170-200 nucleotides. Our results indicate that sequences complementary to the "leader" region and coding portion of 16S mRNA are located in separate parts of the simian virus 40 genome.

Methylated simian virus 40-specific RNA from nuclei and cytoplasm of infected BSC-1 cells

Host cell and virus-specific poly(A)-containing RNAs isolated from nuclei and cytoplasm of monkey kidney cells infected with simian virus 40 contain different methylated nucleotides. In the cytoplasmic simian virus 40-specific RNA, about 75% of the radioactivity derived from (methyl-3-H)methionine was in N-6-methyladenosine (N-6mA) after digestion with Penicillium nuclease and bacterial alkaline phosphatase. The remainder was in a negatively charge component with properties of 5'-terminal structures, i.e., digestion with nucleotide pyrophosphatase and bacterial alkaline phosphatase released 2'-O-methyladenosine (A-m), 2'-O-methylguanosine (G-m), and 7-methylguanosine (m-7-G), consistent with a 5'-terminal structure of the type, m7-GpppNm. The nuclear virus-specific RNA contained N6mA, GM, 2'-O-methyluridine (U-m), and a smaller proportion (10%) of nuclease-, phosphatase-resistant presumptive 5' termini that also yielded A-m, G-m, and m7-G upon further hydrolysis. The infected cell nuclear and cytoplasmic RNAs that did not hybridize to DNA of simian virus 40 contained all four 2'-O-methylnucleosides. The possible role of methylation in the processing and translation of simian virus 40-specific mRNA is discussed.

Synthesis of all the gene products of the reovirus genome in vivo and in vitro

Sixteen virus-specific polypeptides have been resolved in reovirus-infected mouse L cells by using SDS-polyacrylamide slab gel electrophoresis and autoradiography. Of these, ten have been designated as primary products of the genome by the following criteria: they are present in lysates of infected cells labeled for a short time; they co-migrate on SDS-polyacrylamide slab gels with polypeptides synthesized in cell-free-extracts of wheat germ in response to purified viral mRNA; and their molecular weights correspond to the values expected if all ten reovirus mRNA species are monocistronic. Reovirus mRNA species lack 3' poly(A) but are translated into proteins of the expected size. The pattern of synthesis of the primary gene products observed in vitro mimicks that observed in reovirus-infected cells suggesting that the structure of the mRNA may profoundly influence its translation. The results further indicate that there is little, if any, exclusively regulatory information in the reovirus genome since both in vivo and in vitro, transcripts of the ten genome segments direct the synthesis of ten polypeptides that presumably correspond to the primary gene products. The expression of the reovirus genome thus appears to be complete.

Acquisition of sequences homologous to host DNA by closed circular simian virus 40 DNA. 3. Host sequences

A preparation of serially passaged simian virus 40 (SV40) DNA, in which at least 66% of the molecules contain covalently linked cellular DNA sequences, was digested to completion with the Hemophilus influenzae restriction endonuclease. Polyacrylamide gel electrophoresis of the digest showed that the majority of the cleavage products migrated as nine classes of fragments, each class defined by a particular molecular weight. These classes of fragments differ in molecular weight from the fragments produced by the action of the same enzyme on plaque-purified virus DNA. Three classes of fragments were present in less than equimolar amounts relative to the original DNA. The remaining six classes of fragments each contain more than one fragment per original DNA molecule. DNA-DNA hybridization analysis (using the filter method) of the isolated cleavage products demonstrated the presence of highly reiterated cell DNA sequences in two of the nine classes of fragments. A third class of fragments hybridized with high efficiency only to serially passaged SV40 DNA; the level of hybridization to plaque-purified virus DNA was low and there was essentially no hybridization with cell DNA immobilized on filters. It is suggested that this class of fragments contains unique host sequences. It was estimated that at least 27% of the sequences in the substituted SV40 DNA molecules studied are host sequences. The majority of these are probably of the nonreiterated type.

Acquisition of sequences homologous to host DNA by closed circular simian virus 40 DNA. II. Further studies on the serial passage of virus clones

Three plaque isolates of SV40 strain 777 and 1 plaque isolate of strain 776 were grown to high-titer stocks and serially passaged, undiluted, in monkey BS-C-1 cells. In each case, the serial passaging procedure resulted in the accumulation of closed-circular SV40 DNA molecules containing covalently linked sequences homologous to reiterated host cell DNA (called substituted virus DNA). The relative yields, at a given passage level, of SV40 DNA with measurable homology to host DNA varied in different sets of serial passages, including passages of the same virus clone. More reproducible yields of substituted viral DNA progeny were obtained when the serial passaging procedure was initiated from earlier passages rather than from the original plaque-purified stock. Fractionation of closed-circular SV40 DNA molecules on alkaline sucrose gadients indicated that the majority of substituted virus DNA molecules are not plaque producers and are slightly smaller in size than plaque-forming DNA molecules which display no detectable homology to host DNA. Evidence that substituted SV40 DNA molecules replicate during serial undiluted passage was obtained from experiments which demonstrated (i) the presence of host sequences in replicative forms of the viral DNA and (ii) the incorporation of (3)H-thymidine into host sequences isolated from the mature substituted virus DNA molecule.

Acquisition of sequences homologous to host deoxyribonucleic acid by closed circular simian virus 40 deoxyribonucleic acid

The synthesis of closed circular simian virus 40 (SV40) deoxyribonucleic acid (DNA) containing sequences homologous to host cell DNA depends upon the conditions under which the cells are infected. When BS-C-1 monkey cells were infected with non-plaque-purified virus at low multiplicity of infection [MOI, 0.032 plaque-forming units (PFU)/cell], little, if any, of the SV40 DNA extracted from the infected cells hybridized to host DNA; but when increasingly higher multiplicities were used (in the range 0.16 to 3,000 PFU/cell), an increasingly greater amount of the extracted SV40 DNA hybridized to host DNA. The same effect was observed when the closed circular SV40 DNA was extracted from purified virions (grown at low and high MOI) rather than from the infected cell complex. When the cells were infected at high MOI with plaque-purified virus (11 viral clones were tested), none of the SV40 DNA extracted from the cells hybridized detectably with host cell DNA. However, plaque-purified virus that was serially passaged, undiluted, induced the synthesis of virus DNA which again showed extensive homology to host DNA. It is suggested that, under certain circumstances, recombination occurs between viral and host DNA during lytic infection which results in the incorporation of host DNA sequences into closed circular SV40 DNA.