The structure of adenovirus transcripts in the nuclei of infected cells has been studied. Evidence is presented for the existence in the steady-state population of nuclear RNA of two classes of adenovirus type 2 (Ad2)-specified large transcripts. The minor class consisted of molecules containing 3′ poly(A), 5′ cap structures and internal methylated nucleotides. Leader sequences at the 5′ end of these large poly(A) transcripts were identified and characterized by S1 hybridization analyses on alkaline agarose. They represented a spectrum of intermediates which ranged from completely unprocessed (5′ end at 16.5) to completely condensed (5′ end at 26.5) forms. S1 hybrid analyses located 3′ ends of large poly(A) nuclear RNA at 39, 50 and 61.5 on the Ad2 map. A fraction of these nuclear molecules consisted of transcripts that had undergone processing at both 3′ and 5′ ends of mRNA-coding regions since S1 bands corresponding to mature mRNAs for late proteins 52,55K, III, pVI and hexon, as well as 72K early protein were detected. The major class (75 to 85%) of large nuclear transcripts was not polyadenylated, but did contain capped leader sequences. These molecules yielded discrete bands, by S1 analysis, whose 3′ termini could be positioned at sites corresponding to 5′ ends of cytoplasmic mRNA (e.g. 47, 50, 51.5 and 66.5). The possible significance of these poly(A)-RNAs to the transcriptional programme of Ad2 is discussed.


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