NineTeen Complex-subunit Salsa is required for efficient splicing of a subset of small first introns and dorsal-ventral patterning

The NineTeen Complex (NTC), also known as Pre-mRNA-processing factor 19 (Prp19) complex, regulates distinct spliceosome conformational changes necessary for splicing. During Drosophila midblastula transition, splicing is particularly sensitive to mutations in NTC-subunit Fandango, which suggests differential requirements of NTC during development. We show that NTC-subunit Salsa, the Drosophila orthologue of human RNA helicase Aquarius, is particularly rate-limiting for splicing of a subset of small first introns during oogenesis, including the first intron of gurken. Consistent with the functional relevance of such splicing defects, germ line depletion of Salsa reduces adult female fertility, being oocyte dorsal-ventral patterning significantly impaired due to an abnormal expression of Gurken. Our results suggest that although Salsa is apparently not critical for overall splicing, during female gametogenesis there is a subset of introns whose efficient removal relies on the function of this enzyme. Previously, it was shown that Drosophila first introns are more likely to be co-transcriptionally-retained than internal and terminal introns. We propose that Salsa is particularly rate-limiting for splicing of small first introns because it minimizes the interference effect of transcriptional initiation factors binding to U1 snRNP, by facilitating U2 snRNP spliceosome function.