Subviral particles which sedimented at 400 and 600 S were identified in reovirus-infected cells. The 600 S particles (immature virions) contained 10 segments of double-stranded RNA, the major capsid polypeptides λ1, λ2, μ2, σ2, σ3, and a small amount of the nonstructural polypeptide μ0. They lacked A-rich oligonucleotides. Kinetic experiments demonstrated that the acquisition of these oligonucleotides by virions is a very late event in viral morphogenesis. The 400 S particles, which contained 10 segments of double-stranded RNA, consisted of two kinds of particles: (1) short-lived intermediates which contained the core polypeptides λ1, λ2 (in reduced amounts), σ2, and polypeptide μ0; and (2) particles that accumulated in infected cells and that contained the core polypeptides only. When observed in the electron microscope, the latter particles ranged in size from 25 to 54 nm. They lacked the spikes present on the surface of viral cores, but possessed surface detail similar to that on viral cores.
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