Nucleotide substitutions in RmInt1 flanking exon sequences. The schematic diagram shows a wild-type precursor RNA containing the RmInt1 intron and its flanking exons with the predicted EBS/IBS interactions. Arrowheads delimit the RmInt1 intron sequence. Below the flanking exons are indicated all nucleotide substitutions tested and their assignment to three main categories according to the following code: bold upper-case letters, nucleotides and sequences that did not alter the wild-type excision efficiency significantly (86-66% wild-type values); upper-case normal letters, nucleotides and sequences that strongly reduced excision efficiency (to 15-10%); lower-case letters, substitutions that blocked excision (excision products undetectable). Boxed nucleotides correspond to simultaneous substitutions.