ALTERNATIVELY SPLICED TRANSCRIPTS OF THE SEX-DETERMINING GENE TRA-2 OF DROSOPHILA ENCODE FUNCTIONAL PROTEINS OF DIFFERENT SIZE

The Drosophila transformer-2 gene (tra-2) is required for female sex determination in somatic cells and for spermatogenesis in male germ cells. We studied the organization of the tra-2 gene and characterized the transcripts in wild type and mutant animals. Two transcripts are detected in males and females; they differ in their abundance and in the presence (minor transcript T(min)) or absence (major transcript T(maj)) of one exon. Two other transcrips are present only in male germ cells. One of these is rare (msT(min)) and represents a spliced form of the other, more abundant transcript (msT(maj)). The transcript T(maj) encodes a protein of 264 amino acids, whereas transcripts T(min) and msT(maj) encode proteins that are truncated at the N-terminus. All three putative proteins contain a stretch of ~90 amino acids, the ribonucleoprotein motif (RNP motif), which shows similarity to a variety of different ribonucleoproteins. Transformation studies reveal that a cDNA corresponding to the transcript T(maj) can provide all the functions for female sex determination and male fertility. Surprisingly, a cDNA corresponding to the transcript msT(maj) could only supply some female sex-determining function, but was unable to restore fertility in mutant males. Sequence analysis of two temperature-sensitive mutations provides evidence that the RNP motif represents an important functional domain of the tra-2 protein.