Polypeptide components of virions, top component and cores of reovirus type 3

Comparison of the virion polymerase of reovirus with the enzyme purified from reovirus-infected cells

Reovirus has in its protein coat an enzyme which catalyzes the net synthesis of the three size classes of virus-specific, single-stranded ribonucleic acid (RNA). For synthesis of 24, 19, and 14S single-stranded RNA, Mn(++) was the preferred divalent cation, and ammonium sulfate at an optimal concentration of 4.2% of saturation was an absolute requirement. During synthesis, the parental double-stranded RNA was conserved in the viral core and the newly synthesized completed RNA chains were released as free RNA. The viral cores synthesizing RNA had properties consistent with the presence of nascent RNA on their outer surface. The enzyme-template complex from the infected cells described in an earlier paper was comprised of viral cores already active in the in vivo synthesis of single-stranded RNA. This pool of viral cores was newly made during infection, and exponential increase in the number of particles in this pool, as detected by the increase in enzymatic activity, occurred 2 hr earlier than that in mature virus.

Appearance of defective virions in clones of reovirus

Virus obtained from five plaques of reovirus was serially passaged in L cells. Defective virions arose in each clone by the seventh passage. Such defective virions lacked the largest of the 10 segments of the double-stranded ribonucleic acid genome.

The reovirus replicative cycle: conservation of parental RNA and protein

The fate of parental reovirions in the viral replicative cycle has been analyzed using CsCl density centrifugation. After penetration of L-cells, reovirus is converted from a particle of density 1.39 g/cm(3) to a subviral particle of density 1.41 g/cm(3). This alteration in density is temporally correlated with the hydrolysis of viral coat proteins and is qualitatively similar when particles are labeled in their RNA or protein. Ten hours after infection, when synthesis of progeny virus is underway, the parental RNA and protein are again found at density 1.39 g/cm(3). These data demonstrate conservation of the parental RNA and protein in the subviral particle throughout the replicative cycle.

Presence of nucleoside triphosphate phosphohydrolase activity in purified virions of reovirus

A nucleoside triphosphate phosphohydrolase activity has been discovered in reovirus virions. This activity converts nucleoside triphosphates to nucleoside diphosphates in vitro. Properties of this enzyme are presented, with evidence that this activity is an integral part of the virion core.

Defective virions of reovirus

When purified preparations of stock reovirus, type 3, were digested with chymotrypsin, the virions were converted into two different types of particle. These new particles could be separated from each other by isopycnic centrifugation in cesium chloride gradients. One particle banded at a buoyant density of 1.43 g/cm(3), the other at a density of 1.415 g/cm(3). The former particle is termed the heavy (H) particle, the latter is the light (L) particle. The ratio of H/L particles varied between 0.5 and 0.25 in various purified preparations of virus. In electron micrographs, both H and L particles had the appearance and dimensions of viral cores. H particles were infectious for L cells. When plaques formed by stock virus, or by H particles, were picked and propagated in L cells, the majority of the clones gave rise only to H particles on chymotrypsin digestion. On continued serial passage of the clones, virions containing L particles again appeared in the progeny. The simplest explanation of these results was that stock virus was comprised of two populations of virions. One type of virion which contained H particles was infectious, whereas the other, which contained L particles, was not itself infectious and could replicate only in cells coinfected with an H particle virion. Added weight was given to this hypothesis by two observations. First, a small but definite separation of H and L virions could be achieved by isopycnic centrifugation in a gradient of cesium chloride. Second, L particles and virions containing L particles were both shown to lack the largest of the ten segments of double-stranded ribonucleic acid genome. Thus, L particle virions have defective genomes.

Properties of RNA transcriptase in reovirus subviral particles

Subviral particles containing reovirus RNA transcriptase have been isolated from extracts of virus-infected mouse fibroblast cells. The purified particles which lacked the outer protein capsomeres of the mature virion had a buoyant density of 1.43-1.44 g/ml in CsCl and contained all of the double-stranded RNA genome of the intact virus. The particles were free of nuclease activity. RNA synthesis required all four ribonucleoside triphosphates and was dependent on magnesium or manganese; optimal activity required potassium or ammonium ions. In the presence of a ribonucleoside triphosphate regenerating system, reaction rates were linear for 20 hr. RNA yields of 40-fold in excess of input template could be obtained. Completed RNA chains were released from the subviral particles. In the course of RNA synthesis, the double-stranded RNA template was fully conserved. The RNA products formed in vitro displayed profiles in sucrose gradients similar to those found for in vitro reovirus mRNA. The RNA products were single-stranded and did not self-anneal. Over 90 percent of the transcriptase products could be annealed with template double-stranded RNA. The annealed products migrated in acrylamide gels as double-stranded RNA, indicating efficient in vitro transcription.

Transcription in vitro by reovirus-associated ribonucleic acid-dependent polymerase

Digestion of purified reovirus type 3 with chymotrypsin degrades 70% of the viral protein and converts the virions to subviral particles (SVP). The SVP contain 3 of the 6 viral structural proteins and all 10 double-stranded ribonucleic acid (RNA) genome segments but not adenine-rich, single-stranded RNA. An RNA polymerase which is structurally associated with SVP transcribes one strand of each genome segment by a conservative mechanism in vitro. The single-stranded products include large (1.2 x 10(6) daltons), medium (0.7 x 10(6) daltons), and small (0.4 x 10(6) daltons) molecules which hybridize exclusively with the corresponding genome segments. The enzyme obtained by heating virions at 60 C synthesizes similar products. Kinetic and pulse-chase studies indicate that the different-sized products are synthesized simultaneously but at rates which are in the order: small > medium > large.