Abstract: [Objective] To explore the expression pattern of SRY-box transcription factor 5 （SOX5） in different physiological stages of the mammary gland in Chinese Holstein cows, and to clarify its protein structure characteristics and its regulation role in milk fat synthesis. [Methods] Quantitative real-time reverse transcription PCR （qRT-PCR） was used to detect the expression of SOX5 mRNA in bovine mammary gland tissues during the puberty, lactation and dry periods and mammary epithelial cells treated with sodium acetate and oleic acid. RNA interference technology was used to silence SOX5, combined with lipid droplet staining and triglyceride （TAG） content determination to analyze its effect on milk fat synthesis. Bioinformatics methods were performed to analyze the homology, physicochemical properties of the protein, secondary/tertiary structure, conserved domains and interacting protein of the SOX5 gene. [Results] The expression of SOX5 in mammary gland tissue during lactation period was extremely significantly higher than that in puberty and dry periods （P<0.01）. Treatment with 12 mmol/L sodium acetate or 100 μmol/L oleic acid extremely significantly up-regulated the expression of SOX5 in mammary epithelial cells （P<0.001）. After silencing SOX5, the expression of PPARγ （P<0.001）, a key transcription factor of milk fat synthesis, and the expression of SREBP1 （P<0.01） were extremely significantly down-regulated; the number and size of lipid droplets were reduced, and the content of TAG was extremely significantly reduced （P<0.01）. SOX5 is highly conserved among species and has the highest homology with goat （99.3%）. It contains a HMG-box domain and is a soluble hydrophilic protein that can interact with SOX6, SOX9, SATB2 and other proteins. [Conclusion] SOX5 is highly expressed in dairy cows during lactation period and is induced by fatty acids, which can promote the synthesis of lipid droplets and triglycerides. This study provides an important basis for analyzing the regulation mechanism of bovine milk fat synthesis.

Key words: Chinese Holstein cow, SOX5 gene, milk fat synthesis, mammary epithelial cells, bioinformatics