What is the primary function of the Tra protein in Drosophila?

The primary function of the Tra protein in Drosophila is to activate splicing of the Dsx gene. Tra, or transformer protein, plays a crucial role in the sex determination pathway by regulating alternative splicing of the doublesex (Dsx) pre-mRNA. In females, Tra interacts with another protein called Tra2, facilitating the splicing of Dsx pre-mRNA to produce a female-specific isoform of the Dsx protein. This protein then directs the development of female characteristics.

In contrast, in males, the absence of Tra leads to a different splicing outcome of the Dsx pre-mRNA, resulting in a male-specific isoform. Therefore, Tra essentially helps to ensure that each sex develops the appropriate sexual phenotype by activating the correct splicing mechanism for the Dsx gene.