Function of the 16 nt exonic element in the context of AdML pre-mRNAs. In both panels the RNAs are indicated by symbols on the left (triangles indicate a possible exonucleolytic degradation product previously seen with this substrate; 55). (A) Splicing reactions were for 120 min. Control pre-mRNAs (lanes 2 and 4) ended at the BamHI run-off site (PAR) or a GUGAGU 5′ splice site (PAR5c), while heterologous pre-mRNAs (lanes 3 and 5) contained the 16 nt element alone or a 5′ splice site (REP and REP5c). Lane 1, markers. (B) Splicing is silenced by the 16 nt element, but polyadenylation is enhanced. Pre-mRNAs ending in AUUAAA were incubated under splicing conditions for 60 min (lanes 3 and 5) or kept on ice (lanes 2 and 4), in the presence of cordycepin. Parallel reactions, under conditions unfavourable for splicing (lanes 7 and 8), were without cordycepin. The polyadenylation smear is indicated by the bracket and the letter a. Lanes 1 and 6, markers.