A mouse oocyte-specific protein that binds to a region of mZP3 promoter responsible for oocyte-specific mZP3 gene expression

The gene encoding mZP3, the mouse sperm receptor, is expressed exclusively in growing oocytes during oogenesis. To investigate the molecular basis of oocyte-specific mZP3 gene expression, we generated several lines of mice harboring a transgene that contains 470 bp of mZP3 gene 5′-flanking sequence (nucleotides -470 to +10) fused to the firefly luciferase gene coding region. Three of four expressing transgenic lines exhibited luciferase activity only in growing oocytes, suggesting that the 470-bp fragment is sufficient to direct oocyte-specific expression of the luciferase gene. Results of DNase I footprinting and gel mobility shift assays suggested the presence of an ovary-specific protein that binds to a small region (nucleotides -99 to -86) within the 470-bp fragment of the mZP3 promoter, with 5′-G(G/A)T(G/A)A-3′ representing the minimal sequence required for binding. Southwestern (DNA-protein) gel blots revealed the presence of an oocyte-specific, ∼60,000-M_r protein, called OSP-1, that binds to the minimal sequence. Changes in levels of OSP-1 during oogenesis and early cleavage are consistent with the pattern of mZP3 gene expression during these developmental stages in mice. Therefore, OSP-1 may be a mammalian oocyte-specific transcription factor involved in regulating oocyte-specific mZP3 gene expression.