Reovirus replication cycle

Some features of the replication of reovirus in the infected cell are shown in Fig. 15.11. After attachment and entry into the host cell cytoplasm via receptor-mediated endocytosis, reovirus particles are partially uncoated, leaving behind an inner-core subviral particle. This subviral particle contains the 10 genome segments and transcriptional enzymes. Production of mRNAs occurs by the copying of one strand of each duplex genome into a full-length strand. The mRNAs are capped and methylated by viral enzymes but do not have polyadenylated 3' termini. These transcriptional events require six viral enzymes, including a polymerase, a helicase, an RNA triphosphatase, a guanyltransferase, and two distinct methyltransferases. The latter three enzymes are all involved in the capping reaction.

Reovirus

Reovirus

Image Replication Cycle Reovirus

Fig. 15.11 The reovirus replication cycle. Virus attachment is followed by receptor-mediated endocytosis. Virion "core" particles are formed by the degradation of the outer shell in the endosome, and this core particle expresses capped mRNA using a virion transcriptase. Various viral proteins are translated and structural proteins assemble around newly synthesized viral mRNA. This process is apparently random, since random assortment of genetic markers following mixed infection is readily observed (see Chapter 3, Part I). The complementary strand of the double-stranded genomic RNAs is synthesized in the immature capsid while morphogenesis proceeds. Virus release is by cell lysis.

Transept within core ^

Encapsidation of capped (+)-sense RNA

Transept within core ^

Capped mRNAs

Encapsidation of capped (+)-sense RNA

Fig. 15.11 The reovirus replication cycle. Virus attachment is followed by receptor-mediated endocytosis. Virion "core" particles are formed by the degradation of the outer shell in the endosome, and this core particle expresses capped mRNA using a virion transcriptase. Various viral proteins are translated and structural proteins assemble around newly synthesized viral mRNA. This process is apparently random, since random assortment of genetic markers following mixed infection is readily observed (see Chapter 3, Part I). The complementary strand of the double-stranded genomic RNAs is synthesized in the immature capsid while morphogenesis proceeds. Virus release is by cell lysis.

Each of the genome segments encodes a single transcript that is translated into a single protein, except for one of the smaller segments (S1) of the Orthoreovirus genus. This segment encodes two proteins encoded in two nonoverlapping translational reading frames. Both proteins are encoded by the same mRNA by virtue of random recognition of either of the two translation initiation codons by cellular ribosomes. Most of the gene products are structural, either forming one of the multiple capsids or comprising the transcriptional complex of enzymes found within the core.

Replication of the double-stranded genomes and final assembly of progeny virions is not completely understood. It is thought that 10 unique mRNAs associate to form a core progeny virion, associating with the appropriate capsid proteins. These positive-sense RNAs then serve as templates for the synthesis of negative-sense strand, leading to the production of progeny double-stranded genomes within the nascent particle.

This rather convoluted means of generating the double-stranded genome is a consequence of the fact that dsRNA will not readily serve as a template for its own synthesis because of its very great stability. The environment inside the capsid is apparently relatively nonaqueous, and in this nonpolar space, the dsRNA is more readily denatured due to charge repulsion between the phosphate backbones of the two RNA strands. Thus, the double-stranded genome is able to partially denature to serve as a template to generate large quantities of positive-sense mRNA that is extruded from the inner core.

Replication of reovirus RNA, then, does not involve RI-1 or RI-2 intermediates. Further, ideally, no free dsRNA is formed inside the cytoplasm of the infected cell, precluding the induction of interferon. In practice, however, this situation is not realized, and many cells infected with reovirus produce significant interferon. While the yield of virus is quite sensitive to the interferon-mediated antiviral state in cells, apparently the major induction occurs rather late in the replication cycle where cellular organization is deteriorating. Thus, the virus is able to keep ahead of the response for a period of time sufficient for efficient replication in the host.

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