Evidence for a role of the Drosophila melanogaster suppressor of sable gene in the pre-mRNA splicing pathway.

Recessive mutations of the Drosophila melanogaster suppressor of sable [su(s)] gene result in elevated accumulation of RNA from vermilion (v) mutant alleles that have an insertion of the 7.5-kb retrotransposon 412 in the first exon of the v gene. During transcription of such a v mutant gene, the 412 sequences are incorporated into the primary transcripts and are subsequently removed by splicing at cryptic sites within 412 sequences. In a su(s)+ background, the level of these unusually spliced transcripts is exceedingly low, and su(s) mutations increase their accumulation. We previously proposed that v RNA levels are elevated in su(s) mutants because of increased recognition of the cryptic splice sites, and the aim of this study was to test this hypothesis. We generated a v mutant derivative with a smaller 412 insertion, introduced alterations into the 412-associated splice sites, and examined the effect of su(s) mutations on expression of these derivatives after germ line transformation. To increase overall expression levels, the v promoter was replaced with the stronger Metallothionein (Mtn) gene promoter. We found that transformants bearing a v derivative with 480 bp of 412 sequences accumulate both transcripts, with 412 sequences spliced out and transcripts that retain 412 sequences. Mutations of su(s) increase the levels of both transcript classes without affecting the relative amounts of the two forms. Strikingly, replacement of the cryptic 5' splice sites with a 5' consensus produces the same effect as, and eliminates the response to, a su(s) mutation. In addition, we demonstrated that mutations of su(s) lead to increased accumulation of v transcripts even when the previously identified cryptic 412 5' and 3' splice sites were destroyed and that other cryptic splice sites reside within Mtn and 412 sequences. These results indicate that the v mutant transcripts are stabilized by assembly of the 412 sequences into splicing complexes and support the hypothesis that splicing complexes more readily assemble on cryptic splice sites in su(s) mutants.