Those magnificent molecular machines: logistics in dsRNA virus transcription

Double‐stranded (ds) RNA viruses face two major challenges when infecting a cell: the cellular replicative machinery does not operate on dsRNA genomes, and dsRNA provokes a strong apoptotic response. To overcome these problems, dsRNA viruses conceal their genomes from the host cell in an enclosed icosahedral viral core, but then need to carry with them all the necessary enzymatic requirements for replication and transcription. dsRNA virus genomes are often segmented (up to 12 segments). Whereas the transcription and capping reactions have been well documented (see, for example, Luongo et al ., 2000; Reinisch et al ., 2000), the mechanism used for genome packaging, and the site for minus strand synthesis, have not been well defined. A paper by Diprose et al . (2001) now sheds new light on the trafficking of the precursors and transcription products of an active dsRNA virus core. These cores are symmetrical (icosahedral) particles with 2‐, 3‐ and 5‐fold axes of symmetry (Figure 1; Grimes et al ., 1998).Figure 1. A schematic illustration of a dsRNA virus genome replication cycle. The infection process brings into the cell the core containing the dsRNA genome segments ( a ). Upon uncoating, the particle is activated to synthesize single‐stranded (ss) RNA copies from each genome segment. These exit through machinery occupying the icosahedral particle vertices ( b ). The ssRNA segments are of positive polarity and are utilized as mRNAs ( c ), leading to the synthesis of proteins that …