Tissue specificity of Drosophila P element transposition is regulated at the level of mRNA splicing.

We show that the germline specificity of P element transposition is controlled at the level of mRNA splicing and not at the level of transcription. In the major P element RNA transcript, isolated from somatic cells, the first three open reading frames are joined by the removal of two introns. Using in vitro mutagenesis and genetic analysis we demonstrate the existence of a third intron whose removal is required for transposase production. We propose that this intron is only removed in the germline and that its removal is the sole basis for the germline restriction of P element transposition.