Human rotaviruses (family Reoviridae) are responsible for the deaths of over 600,000 children worldwide each year. They cause severe diarrhea, which rapidly causes dehydration and death if appropriate therapy is not provided. Because of their impact on humans, rotaviruses have been studied intensely to better understand their life cycles and pathogenesis. Gastroenteritis (viral)
Viewed by electron microscopy, rotavirus virions have a characteristic wheel-like appearance (rotavirus is derived from the Latin rota, meaning wheel). Virions are nonenveloped and are composed of 11 segments of dsRNA surrounded by three concentric layers of proteins (see figure).
The RNA segments code for six structural and six nonstructural proteins. When a rotavirus virion enters a host cell, it loses the outermost protein layer and is then referred to as a double-layered particle (DLP; figure 27.21). The genome is transcribed by the viral transcriptase while still inside the DLP. The mRNA passes through channels in the DLP and is released into the cytosol of the host cell.
There the mRNAs are translated by the host cell’s protein synthesis machinery. The newly formed proteins cluster together, forming an inclusion called a viroplasm. It is within the viroplasm that new DLPs are formed. Initially, the DLPs contain plus-strand RNA, but this is soon used as a template for the synthesis of the negative strand.
VP1-8 are viral proteins that become part of the virion. NSP4 is a nonstructural protein that functions in virion assembly but is not incorporated into the virion. The method of release of progeny virions has not been definitively determined.
Thus the dsRNA molecule is synthesized within the developing DLP. DLPs containing dsRNA eventually leave the viroplasm and are enveloped by membranes of the endoplasmic reticulum. While in the endoplasmic reticulum, the outermost layer is added to the DLP, converting it to the mature triple-layered virion. The mature virion is released by an unknown mechanism.
Double-Stranded RNA Viruses: RNA-Dependent RNA Polymerase Replicates the Genome and Synthesizes mRNA
■ Double-stranded RNA viruses use a viral enzyme called RNA-dependent RNA polymerase to synthesize mRNA (transcriptase activity) and replicate their genomes (replicase activity) (figure).
■ Bacteriophage f6 is an unusual phage in that it is enveloped and enters the host bacterium through a process that resembles endocytosis. It has a segmented genome, in which each segment encodes a polycistronic mRNA that yields several proteins.
■ Rotaviruses have segmented genomes, in which each segment codes for one or two proteins. They multiply in an inclusion called the viroplasm (figure).