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The Sry gene functions as a genetic switch initiating testicular development of the indifferent mammalian gonad. The Mus musculus molossinus Sry open reading frame (ORF) encodes a 395-amino acid transcription factor (mSry) that specifically binds and bends DNA through its N-terminal HMG domain and activates transcription through its long C-terminal (residues 144-366) glutamine/histidine-rich activation domain. The M. m. domesticus Sry ORF encodes a highly homologous, truncated protein (dSry) of approximately 230 amino acids, and the molecular basis for truncation is a point mutation that creates an amber stop codon within the activation domain. The mSry protein activates transcription of a Sry-responsive reporter gene in HeLa cells, but dSry does not. Gene swapping and in vitro DNA binding experiments revealed that lack of transcriptional activation by dSry was not the result of polymorphisms within the first 137 amino acids of the protein. Direct analysis of the C-terminal glutamine/histidine-rich domain revealed that dSry lacked a functional transcriptional activation domain. Fusion of the GAL4 DNA-binding domain to the C-terminal deletion mutants of the GAL4-mSry chimeric protein indicated that residues 263-345 of the glutamine/histidine-rich domain were necessary for high level transactivation. Furthermore, readthrough of the premature amber stop codon by transfer RNA suppression resulted in a strong GAL4-dSry transactivator. This demonstrated that the premature stop codon is the only polymorphism responsible for the inability of the dSry glutamine/histidine-rich region to transactivate.

The SRY gene functions as a genetic switch in gonadal ridge initiating testis determination. The mouse Sry and human SRY open reading frames (ORFs) share a conserved DNA-binding domain (the HMG-box) yet exhibit no additional homology outside this region. As judged by the accumulation of lacZ-SRY hybrid proteins in the nucleus, both the human and mouse SRY ORFs contain a nuclear localization signal. The mouse Sry HMG-box domain selectively binds the sequence NACAAT in vitro when challenged with a random pool of oligonucleotides and binds AACAAT with the highest affinity. When put under the control of a heterologous promotor, the mouse Sry gene activated transcription of a reporter gene containing multiple copies of the AACAAT binding site. Activation was likewise observed for a GAL4-responsive reporter gene, when the mouse Sry gene was linked to the DNA-binding domain of GAL4. Using this system, the activation function was mapped to a glutamine/histidine-rich domain. In addition, LexA-mouse Sry fusion genes activated a LexA-responsive reporter gene in yeast. In contrast, a GAL4-human SRY fusion gene did not cause transcriptional activation. These studies suggest that both the human and the mouse SRY ORFs encode nuclear, DNA-binding proteins and that the mouse Sry ORF can function as a transcriptional activator with separable DNA-binding and activator domains.