Messenger RNA splicing in vitro: an excised intervening sequence and a potential intermediate.

Four RNA products have been characterized from a soluble system that accurately splices purified substrate RNA. The labeled substrate RNA contained the first and second exons of the major late transcription unit of adenovirus 2 and a shortened form of the first intervening sequence. One of these RNA products was the excised intervening sequence which accumulated quantitatively with the level of splicing. This RNA species has an unusual structure and is modified at internal sites. A potential intermediate in the splicing pathway which is cleaved at the 5' splice site of the first exon has been isolated. This RNA species is also modified at sites within the first intervening sequence. These results suggest that the splicing of mRNA precursors may involve sites in the intervening sequence, cleavage at the 5' splice site, cleavage at the 3' splice site, and ligation of the two exons.