The molecular biology of reovirus

The genome of reoviruses consists of ten discrete segments of double-stranded RNA. The molecular weight of three of these, the L segments, is about 2.7 × 10⁶; of another three, the M segments, about 1.4 × 10⁶; and of the remaining four, the S segments, 0.6–0.8 × 10⁶. In addition, reovirions contain about 2,000 molecules of oligonucleotide from 2 to 20 nucleotides long, most of which are very rich in ademine. The protein capsid of reovirions, which consists of an inner core and 92 outer shell capsomeres, is composed of seven species of polypeptides which also fall into three size classes. The size of six of these polypeptides is such that each accounts for the entire information content of one of the genome RNA segments. Reovirus messenger RNA molecules are transcripts of all ten genome RNA segments. Transcription proceeds not only in vivo in the infected cell, but can also take place in vitro. It is then catalyzed by reovirus cores, the polypeptide composition of which is strikingly similar to that of E. coli DNA-dependent RNA polymerase. Transcription both in vitro and in vivo is entirely asymmetric and is conservative with respect to template. The patterns of transcription of the several reovirus genome RNA segments have been determined both in vitro and in vivo and the pattern of translation has been analyzed under in vivo conditions. The results suggest that transcription and especially translation are regulated within infected cells.