Mechanism of alternative splicing regulation of Drosophila P-element somatic inhibitor (PSI) protein. (A) Productive splicing of Drosophila P-element pre-mRNA in germ-line cells leads to the translation of an 87-kDa transposase protein. Alternative splicing in somatic cells leads to inclusion of intron 3. A stop codon (○) located in intron 3 prevents translation of the full-length transposase protein, resulting in a truncated 67-kDa protein that acts to repress transposition. (B) In germline cells, U1 snRNP (U1) binds to the actual 5′-ss (AS) or the F1 pseudo-splice site with approximately equal affinity. PSI, which binds to the F2 pseudo-splice site, recruits U1snRNP to the F1 pseudo-splice site in addition to preventing U1snRNP binding to the actual site, thus preventing spliceosome formation.
