NineTeen Complex-subunit Salsa is required for efficient splicing of a subset of 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 rate-limiting for splicing of a subset of small first introns during oogenesis, including the first intron of gurken. Germ line depletion of Salsa and splice site mutations within gurken first intron both impair adult female fertility and oocyte dorsal-ventral patterning due to an abnormal expression of Gurken. Supporting causality, the fertility and dorsal-ventral patterning defects observed after Salsa depletion could be suppressed by the expression of a gurken construct without its first intron. Altogether our results suggest that one of the key rate-limiting functions of Salsa during oogenesis is to ensure the correct expression and efficient splicing of the first intron of gurken mRNA. Retention of gurken first intron compromises the function of this gene most likely because it undermines the correct structure and function of the transcript 5'UTR.