Mouse beta-globin and adenovirus-2 major late transcripts are initiated at the cap site in vitro.

Initiation of transcription on major late DNA of adenovirus serotype 2 and the beta-globin gene of the mouse was studied by using ribonucleoside [beta-32P]triphosphates as labeled precursors in vitro. The 5' termini of discrete runoff transcripts of both templates were preferentially labeled. The beta phosphate of the cap structure was labeled by [beta-32P]ATP but not [beta-32P]GTP, indicating that the beta phosphate of the cap is contributed by the initiating nucleoside triphosphate (ATP in both cases). Therefore, capping occurs on the initial nucleotide of these transcripts.