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The molecular size of ‘early’ and ‘late’ SV 40-specific RNA synthesized in productively infected monkey kidney cells was investigated. Labelled RNA extracted from infected cells was fractionated by velocity centrifugation in a sucrose density gradient and then hybridized with excess, unlabelled SV40 DNA. The largest component in preparations of ‘early’ SV 40-specific RNA, which was synthesized in actinomycin D-treated cells, had a molecular weight of approximately 1·2 × 106. Late after infection, the molecular weight of the predominant species of cytoplasmic SV 40-specific RNA was about 1·7 × 106. The latter molecular size corresponds to the full transcription of one SV 40 DNA strand into a polycistronic messenger RNA. The major portion of nuclear ‘late’ virusspecific RNA sedimented at 32 to 50s (molecular weight range: 2·3 to 5·7 × 106) , indicating that in the nuclei of productively infected cells the virus genome can be transcribed into RNA molecules much larger than a single virus DNA strand.
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