Construct and processing pathway of pre-18S RNA. The pre-18S RNA construct spans the transcription start site to position 239 of ITS1 and is expressed under a GAL7 promotor from plasmid pLM104. The location of hybridization probes are indicated for three plasmid-specific sequences: a MS2 coat protein binding motif at the 5′ end of 5′ ETS, a neutral sequence (18S-tag) inserted between positions 232 and 233 of 18S, and a tobramycin-binding aptamer (TOB) between positions 152 and 153 of ITS1. By analogy to processing of 23S pre-rRNA, the pre-18S RNA is expected to be sequentially cleaved at the A0, A1, A2, and D sites, yielding 22S-, 21S-, and 20S-like intermediates.
