Molecular cloning of a novel mRNA sequence expressed in cleavage stage mouse embryos

In an approach to study genes transcribed during early mouse development, a cDNA library was constructed from poly(A) RNA isolated from the 8-cell morula. The cDNA library was differentially screened with labelled cDNA probes synthesized on poly(A) RNA isolated from the 8-cell morula or unfertilized eggs. Six clones which increased in abundance in the 8-cell morula were selected and further analyzed. Sequencing analyses showed that some of these clones corresponded to RNA transcripts from B1 and B2 repetitive sequences, as well as mRNA for cytochrome C oxidase I and NADH dehydrogenase III derived from the mitochondrial genome. One clone was not identical to any known sequences. The unidentified sequence (MO25) was found at low levels in the unfertilized egg, but increased at the 2-cell stage. The predicted amino acid sequence revealed that the MO25 gene may encode a Ca2+ binding protein.