EFFICIENT TRANSSPLICING OF MUTATED SPLICED LEADER EXONS IN LEISHMANIA-TARENTOLAE

Every kinetoplastid mRNA receives a common, conserved leader sequence via the process of trans-splicing. In Leishmania tarentolae the precur sor spliced leader RNA is 96 nucleotides, with a 39-nucleotide exon th at is 7meG-capped and methylated on the first 4 nucleotides. trans-Spl icing was inferred from the presence of tagged leader in the high mole cular weight RNA population and confirmed for accuracy by cDNA cloning . Linker scan substitutions within the exon between positions 10 and 3 9 did not affect trans-splicing. The trans-splicing efficiency for thr ee of the scan exons was proportional to the tagged:wild type ratio in the spliced leader precursor population. Two scan leader RNAs that we re efficiently spliced showed reduced methylation. Longer exons showed reduced splicing, whereas 10- or 20-base pair deletions abolished spl icing. These results indicate that size, but not content, of the exon is a constraint on the splicing process. These results, in combination with previous data eliminating a role in transcription initiation, su ggest that translation may be the selective pressure on the leader con tent.