FIG. 1.

Expression system used to analyze mutant leader RNAs. Shown is a schematic of the U2-SL1 construct. Fragments (∼240 nt) of each mutant SL RNA gene, including 135 nt of 3′ nontranscribed sequence (used to ensure correct 3′ end formation), were cloned directly behind a 340-nt fragment containing the U2-3 snRNA promoter. This latter fragment contains the C. elegans U snRNA consensus proximal sequence element sequence (beginning 65 nt before the start site of transcription [38]), which is likely to be an important transcriptional regulatory element, based on studies in other systems (33).