Molecular genetics of androgen-dependent and -independent expression of mouse sex-limited protein.

Genes of the mouse S locus encoding C4 (the fourth component complement) and Slp (sex-limited protein) show extensive homology but are distinct in their function and regulation. In some mouse strains, such as B10.D2, Slp is androgen regulated, whereas in others, such as B10.W7R, expression of Slp is constitutive. We have previously shown that the B10.W7R strain has multiple Slp genes. In this report, we present the structure of the single C4 and four Slp genes of the B10.W7R S locus and compare the upstream flanking regions by partial sequence analysis and function in transfection assays. Of the four Slp genes, three (Slpw7.A, Slpw7.B, and Slpw7.C) have upstream and promoter regions very similar to those of C4. The fourth Slp gene (Slpw7.D) is instead virtually identical to the androgen-regulated allele (Slpd from the B10.D2 mouse) in upstream regions. In particular, far-upstream sequences from both Slpd and Slpw7.D render the bacterial chloramphenicol acetyltransferase gene hormonally responsive upon transfection into mammary carcinoma cell lines. The upstream sequences between 2 to 3 kilobases of the Slp promoter initiate transcription from multiple sites when fused proximal to the chloramphenicol acetyltransferase gene, and these transcripts are threefold more abundant in the presence of androgen. This behavior is similar for Slpd and Slpw7.D, which suggests that Slpw7.D may be androgen regulated but that this is masked in vivo by constitutive expression of the other Slp genes. Nonhomologous recombination is implicated not only in expanding the copy number of C4 and Slp genes in the B10.W7R mouse but also in creating hybrid genes with regulatory features of C4 and structural features of Slp.