Reovirus reverse genetics: Incorporation of the CAT gene into the reovirus genome.

We have modified the infectious reovirus RNA system so as to generate a reovirus reverse genetics system. The system consists of (i) the plus strands of nine wild-type reovirus genome segments; (ii) transcripts of the genetically modified cDNA form of the tenth genome segment; and (iii) a cell line transformed so as to express the protein normally encoded by the tenth genome segment. In the work described here, we have generated a serotype 3 reovirus into the S2 double-stranded RNA genome segment of which the CAT gene has been cloned. The virus is stable, replicates in cells that have been transformed (so as to express the S2 gene product, protein final sigma 2), and expresses high levels of CAT activity. This technology can be extended to members of the orbivirus and rotavirus genera. This technology provides a powerful system for basic studies of double-stranded RNA virus replication; a nonpathogenic viral vector that replicates to high titers and could be used for clinical applications; and a system for providing nonselectable viral variants (the result of mutations, insertions, and deletions) that could be valuable for the construction of viral vaccine strains against human and animal pathogens.

Construction and characterization of a reovirus double temperature-sensitive mutant.

The infectious reovirus RNA system was used to construct a mutant with two temperature-sensitive (ts) lesions in genome segments M2 and S2, respectively. The double mutant is about 300 times more ts than either of its parents, which are about 1,500 and 170 times more ts than their wild-type parent reovirus ST3 strain Dearing. At 39 degrees C the double mutant is essentially unable to multiply. In spite of its striking temperature sensitivity, the double mutant elicits the formation of significant amounts of neutralizing antibodies in newborn mice. Possible mechanisms responsible for this are discussed, as is the significance of this double ts mutant in relation to current searches for safe and efficient vaccine strains.

Identification of signals required for the insertion of heterologous genome segments into the reovirus genome.

In cells simultaneously infected with any two of the three reovirus serotypes ST1, ST2, and ST3, up to 15% of the yields are intertypic reassortants that contain all possible combinations of parental genome segments. We have now found that not all genome segments in reassortants are wild type. In reassortants that possess more ST1 than ST3 genome segments, all ST1 genome segments appear to be wild type, but the incoming ST3 genome segments possess mutations that make them more similar to the ST1 genome segments that they replace. In reassortants resulting from crosses of the more distantly related ST3 and ST2 viruses that possess a majority of ST3 genome segments, all incoming ST2 genome segments are wild type, but the ST3 S4 genome segment possesses two mutations, G74 to A and G624 to A, that function as acceptance signals. Recognition of these signals has far-reaching implications for the construction of reoviruses with novel properties and functions.

Translation of reovirus RNA species m1 can initiate at either of the first two in-frame initiation codons.

The m1 species of reovirus RNA, which encodes the minor protein component mu 2, possesses two initiation codons, one "strong" according to Kozak rules and preceded by 13 residues (IC1), the other "weak" and located 49 codons downstream of the first (IC2). In reovirus-infected cells only IC2 is used, but initiation from IC1 can be activated, and efficiency of initiation from either initiation codon modulated over a wide range, by coupling unrelated sequences to either or both ends of m1 RNA. For example, when the M1 genome segment is cloned into the thymidine kinase gene of vaccinia virus in such a way that various "irrelevant" stretches of nucleotides comprising restriction endonuclease cleavage sites or promoter remnants are coupled to the 5' end of m1 RNA, translation of the resultant transcripts is also initiated at IC2, with frequencies controlled by the nature of the attached sequences. However, in rabbit reticulocyte lysates these same transcripts are translated from IC1 as well as from IC2, and transcripts in which m1 RNA is preceded by long sequences of encephalomyocarditis virus RNA (from the T7 polymerase-controlled pTM1 vector) are translated exclusively from IC1. By contrast, m1 RNA itself is translated only from IC2. It appears that the most important factor that controls the extent to which translation is initiated from IC1 and IC2 is their "availability," which is likely to be a function of the extent to which the regions on either side of them interact with each other (and also, to a lesser extent, with the 3' untranslated region) either directly or via interaction with host cell proteins. The effects described here are of considerable potential significance when genetic material is rearranged as a result of translocations, insertions, deletions, and amplifications--that is, when sequences that are normally separated are brought into apposition.

Reovirus RNA is infectious.

Conditions under which reovirus RNA is infectious have been worked out. In brief, single-stranded (plus-stranded, ss) and/or double-stranded (ds) RNA of reovirus serotype 3 (ST3 virus) is lipofected into L929 mouse fibroblasts together with a rabbit reticulocyte lysate in which ss or melted dsRNA has been translated. After 8 hr the cells are then infected with a helper virus, ST2 reovirus. Virus yields are harvested 24 or 48 hr later. Under these conditions virus that forms plaques by 5 days is produced, all of which is ST3 virus; ST2 virus forms plaques only after 12 days. No reassortants are present among the progeny. The virus yields are about 0.2 PFU/cell; immunofluorescence assays show that this progeny is derived from about 4% of the cells. Double-stranded RNA is 20 times as infectious as ssRNA; ds and ssRNA together yield 10 times as much infectious virus as dsRNA alone, the reason being that dsRNA greatly increases the infectiousness of ssRNA. All species of both ss and dsRNA are required for the operation of this additive effect. The primed rabbit reticulocyte lysate is not essential, but increases virus yields by 100-fold. Its activity is proportional to the time for which translation has proceeded; however, this activity is not due solely to newly synthesized proteins because destruction of the RNA following translation abolishes activity which cannot be restored by simple addition of more RNA. Translation of all species of RNA is essential. Whereas no reassortants are formed when ss and dsRNA of different genotypes are lipofected together, mixtures of dsRNAs of different genotypes do yield reassortants. The same is true for such mixtures of ssRNA. These findings will permit the introduction of new or altered genome segments into the reovirus genome. They open the way to the identification of encapsidation and assortment signals on reovirus genome segments, the characterization of functional domains on reovirus proteins, and the development of reovirus as an expression vector.

Control of reovirus messenger RNA translation efficiency by the regions upstream of initiation codons.

The 10 species of reovirus messenger RNA are translated in vivo with efficiencies/frequencies that differ by as much as 100-fold. The s1 mRNA, which is translated 10 times less efficiently than the s4 mRNA but 10 times more efficiently than the/1 and m1 mRNAs, has a unique BamH1 cleavage site located immediately downstream of its initiation codon. Because the reovirus mRNAs have been cloned, this provides the opportunity for placing modified and altered sequences upstream of its coding sequence. The translation efficiencies of the variant mRNAs, transcribed via the SP6 in vitro transcription system, can then be measured in the rabbit reticulocyte lysate in vitro translation system. Using this system it was found that replacing the 5'-upstream sequence of the s1 mRNA with that of the s4 mRNA increases its in vitro translation efficiency by 4-fold; that the trinucleotide immediately upstream of the s1 initiation codon renders it very weak, and that it is only slightly superior to the weakest Kozak consensus sequence; that the nature of the nucleotides further upstream than position -3 can profoundly affect translation efficiency; that the nature of this effect is in turn markedly modified by the nature of nucleotides in positions -1 to -3; and that there is a minimum optimal 5'-upstream sequence length of about 14 nucleotides. We also investigated the effect of secondary structure involvement on the ability of 5'-upstream sequences to promote translation. Two effects were noted. First, being part of moderately stable stem loops (delta G, -18 kcal/mol) decreased translation efficiency about 3-fold; second, mRNA in which only three 5'-terminal nucleotides were unpaired were translated five times less efficiently than mRNA in which six nucleotides were unpaired. Accessibility of the 5'-cap as well as secondary structure of the 5'-upstream sequences are therefore factors that affect translation efficiency. Finally, we showed that the m1 mRNA, which is transcribed very poorly in vivo, is translated very efficiently in vitro; and that its 5'-upstream sequence is as effective in increasing protein sigma 1 formation as that of s4 mRNA. Since both m1 mRNA and protein mu 2 are stable in infected cells, the reason why m1 mRNA is translated so inefficiently in vivo therefore remains unexplained.

Cellular integrity is required for inhibition of initiation of cellular DNA synthesis by reovirus type 3.

Synchronized HeLa cells, primed for entry into the synthesis phase by amethopterin, were prevented from initiating DNA synthesis 9 h after infection with reovirus type 3. However, nuclei isolated from synchronized cells infected with reovirus for 9 or 16 h demonstrated a restored ability to synthesize DNA. The addition of enucleated cytoplasmic extracts from infected or uninfected cells did not affect this restored capacity for synthesis. The addition of ribonucleotide triphosphates to nuclei isolated from infected cells stimulated additional DNA synthesis, suggesting that these nuclei were competent to initiate new rounds of DNA replication. Permeabilization of infected cells did not restore the ability of these cells to synthesize DNA. Nucleoids isolated from intact or permeabilized cells, infected for 9 or 16 h displayed an increased rate of sedimentation when compared with nucleoids isolated from uninfected cells. Nucleoids isolated from the nuclei of infected cells demonstrated a rate of sedimentation similar to that of nucleoids isolated from the nuclei of uninfected cells. The inhibition of initiation of cellular DNA synthesis by reovirus type 3 appears not to have been due to a permanent alteration of the replication complex, but this inhibition could be reversed by the removal of that complex from factors unique to the structural or metabolic integrity of the infected cell.