Life science
Medical & healthcare, Drug discovery, LivestockStudies on the genomic structure and evolution ofthe mammalian sex-determining gene SryLab. of Epigenome Dynamics, Graduate School of Frontier Biosciences
Professor Makoto Tachibana
Researchmap https://researchmap.jp/read0192906

Life science

Abstract
The mammalian sex-determining gene Sry induces male development. Since its discovery 30 years ago, Sry has been believed
to be a single-exon gene. Here, we identified a cryptic second
exon of mouse Sry and a corresponding two-exon type Sry (Sry-T)transcript. XY mice lacking Sry-T were sex-reversed, and ectopic
expression of Sry-T in XX mice induced male development. Sry-T
messenger RNA is expressed similarly to that of canonical single-exon type Sry (Sry-S), but SRY-T protein is expressed predominantly because of the absence of a degron in the C terminus ofSRY-S. Our findings suggest that in nature, SRY-T, not SRY-S, is thebona fide testis-determining factor.

▍ Background & Results
Sexual differentiation is essential for the survival and evolution ofa species. Expression of the Y chromosomal gene Sry is required
for male development in mammals. Since its discovery in 1990, Sry
has been considered a single-exon gene encoding only one protein. Using a comprehensive transcriptomics approach, we identified a transcribed sequence within palindromic sequences flanking
Sry. We identified this sequence as the 3′portion of a novel Srytranscript that starts at the known Sry transcription start site and is
spliced once, indicating that this sequence represents a second
exon of Sry. Therefore, the Sry locus is found to produce two Srytranscripts, the known single-exon type (Sry-S) and a two-exon
type (Sry-T). The predicted SRY-T protein shares amino acids 1
to 377 with SRY-S, including the high-mobility group DNA-binding
domain and polyglutamine (poly-Q) sequences that are important
for transcriptional activation. Beyond this, the C-terminal 15 aminoacids of SRY-T are encoded by the second exon, whereas absenceof splicing results in a different 18 amino acids at the C terminus ofSRY-S. We found that the latter sequence encodes a protein-degradation motif, degron, but the former sequence does not. Protein expression of SRY-T predominates over that of SRY-S, because of theabsence of a degron in the C terminus of SRY-S. XY mice lacking
SRY-T were sex-reversed, and ectopic expression of SRY-T in XX
mice induced male development. Our findings indicate that SRY-T,
not SRY-S, is the bona fide testis-determining factor.

Figure 1. There is a "hidden" second exon in the Sry gene. The Sry geneproduces two mRNA transcripts, the known single-exon type (Sry-S) and a twoexon type (Sry-T).

Figure 2. Differences in sequence and function between SRY-S and SRY-T.

▍ Significance of the research and Future perspective
Our findings overturn the conclusion that has prevailed for 30
years regarding the structure of Sry in the mouse, the most intensively studied mammalian model of sex determination. Further
study of SRY-T will establish whether the second exon confers new
functionalities on SRY in addition to protein stability and will provide a more complete understanding of how the sex-determining
cascade is activated in mice compared with other mammalian species.

Figure 3. XY mouse lacking Sry-T differentiated into female, whereas XX micecarrying Sry-T differentiated into male.

P a t e n t
Miyawaki, Shingo; Kuroki, Shunsuke; Maeda, Ryo et al. The mouse Sry locus harbors a cryptic exon that is essential for male sex determination. Science
2020; 370(6512): 121-124. DOI: 10.1126/science.abb6430
UR
L https://tachibana-lab.net/
K e y w o r d sex-determining gene, evolution, mammal, protein degradation
Tr e a t i s e