Order of polyadenylic acid addition and splicing events in early adenovirus mRNA formation.

A study of the processing of mRNA from two early adenovirus type 2 transcription units (regions 2 and 4 of adenovirus type 2 genome; [J. Flint, Cell 10:153--166, 1977]) revealed that polyadenylic acid [poly(A)] was added in most if not all cases to an unspliced nuclear RNA molecule whose coordinates extended from the apparent initiation site for RNA synthesis to the single poly(A) site in each transcription unit. An intermediate RNA molecule in the processing of the mRNA for the 72,000-M, single-stranded DNA binding protein showed that the first of the two intervening sequences, the one closest to the 5' end of the molecule, was removed first at least in the majority of the processed molecules. Finally, in cells labeled for 10 min and then treated with actinomycin D to stop further RNA synthesis, the majority, if not all, of the poly(A)-terminated nuclear RNA specific for region 2 was successfully processed and transported to the cytoplasm.