Isolation and developmental expression of an oogenesis-specific Xenopus cDNA clone.

We have characterized a cDNA clone designated XSYO, complementary to a transcript that is highly expressed during Xenopus oogenesis. XSYO is expressed as a maternal mRNA in oocytes and early embryos at a level up to 3 × 108 copies per mature oocyte. This level is 100-fold higher than the concentration of an average maternal RNA in the oocyte and close to the value found for the stockpiled maternal histone mRNA. This level remains constant thoughout the first rapid cleavage stages until the midblastula transition (MBT). After this stage, the XSYO RNA is degraded within 2 h and, after the blastula stage, a very weak expression was detected. This gene is not expressed in Xenopus proliferative somatic cultured cells, suggesting that it is not a simple housekeeping gene. The presence of a potential metal-binding domain in the XSYO sequence suggests that this gene might code for a protein involved in nucleic acid binding or gene regulation specific to oogenesis or early development.