Studies on reovirus RNA. II. Characterization of reovirus messenger RNA and of the genome RNA segments from which it is transcribed

Unexpectedly large size of globin messenger ribonucleic acid

The globin messenger RNA of rabbit reticulocytes is identified on the basis of its presence in polysomes, absence from single ribosomes, and sedimentation coefficient of 9 S. Analysis of this RNA by electrophoresis in polyacrylamide gels provides evidence that its size is unexpectedly large (approximately 650 nucleotides) for a messenger coding for the 141 and 146 amino acids of a globin polypeptide. Reticulocyte RNA molecules were labeled in vivo with [(32)P]orthophosphate. Globin messenger RNA and transfer RNA appear to be synthesized late in erythropoiesis, whereas ribosomal RNA and 5S RNA are synthesized at the beginning stages of cytodifferentiation. At least one fragment of ribosomal RNA accumulates in erythrocytes in the terminal stages of maturation.

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.

Structural studies on reovirus: discontinuities in the genome

The double-stranded RNA genome of reovirus breaks reproducibly into ten segments upon extraction from purified virions. Reovirus-induced messenger RNA's formed in infected cells correspond in length to these ten genomic segments and some real structural and biological implication must be accorded to the ready manner in which the genome is fragmented on attempted isolation. In the present work we have posed the question whether the reovirus genome is a continuous, double-stranded molecule or whether there are discontinuities in the complementary strands of RNA that might constitute "weak points" in the structure. To answer this question, a method was developed to estimate the 3'-terminal nucleosides of RNA within intact virions. Approximately as many 3'-ends are free inside the virion as in RNA extracted from purified virus. Thus, the viral genome exists as a discontinuous structure inside the virus particle and both strands of the duplex are interrupted at intervals.

Structural proteins of reoviruses

Polyacrylamide gel electrophoresis of the solubilized proteins from the three serotypes of reovirus revealed that each contained three major and four minor components. Subviral particles were prepared by brief treatment of complete virions with urea. Electron microscopy, density-gradient centrifugation, and chemical analyses of these particles indicated that their outer capsid structure had been selectively removed. They contained only two proteins, but their ribonucleic acid composition was similar to that of complete virions. The subviral particles were not infectious.

Separation of ten reovirus genome segments by polyacrylamide gel electrophoresis

Double-stranded ribonucleic acid (RNA) extracted from purified reoviruses of all three serotypes and from type 3 virus-infected cells was analyzed by polyacrylamide gel electrophoresis. It was calculated that each RNA includes 10 segments: 3 large, 3 intermediate, and 4 small fragments corresponding to molecular weights of about 2.5, 1.4, and 0.8 x 10(6) daltons, respectively, or a total of 15 x 10(6) daltons.

Transcription of the genomes of type 1 and type 3 reoviruses

The double-stranded ribonucleic acid (RNA) genome of type 1 reovirus was fragmented into three size classes on extraction, as has already been shown for reovirus type 3. The relative amounts and molecular weights of the three classes were the same for the two viruses. Cells infected with type 1 virus synthesized three classes of messenger RNA. Each class of messenger RNA hybridized exclusively with a denatured double-stranded RNA fragment of equivalent length, as had also been found for type 3 reovirus. The double-stranded RNA segments thus act as specific units for transcription of messenger RNA in the infected cells. In cells infected with type 3 reovirus, the three classes of messenger RNA are made in equal amounts throughout the course of multiplication. In contrast, cells infected with type 1 virus produced only half as much of the largest messenger RNA as they did of the other two classes at all times during the replicative cycle. This finding suggests that transcription of the largest segments of type 1 viral genome is restricted.