Characterization and expression of a Xenopus ras during oogenesis and development

We have characterized a cDNA which contains the entire coding sequence of a Xenopus laevis ras protein. The deduced amino acid sequence reveals a strong homology (92%) to human Ki-ras 2B protein. ras expression has been studied both qualitatively and quantitatively during Xenopus development. ras is expressed as a maternal mRNA in oocytes and early embryos at a level up to 1.5 x 10(7) copies per mature oocyte, corresponding to the level of ras mRNA found in 4 x 10(5) somatic growing cells. This level remains constant throughout the first rapid cleavage stages of the blastula before the midblastula transition (MBT). After this stage, the amount of ras RNA decreases gradually until the hatching tadpole stage, when a new zygotic expression is detected in the embryo. From that stage, a constitutive amount of 30-50 ras RNA transcripts per embryonic cell is registered, as observed in Xenopus proliferative somatic cells. The 23-kDa Xenopus ras protein has also been identified by both specific monoclonal antibody and in vitro transcription-translation experiments. It is expressed in oocytes before maturation, indicating that maturation is not the trigger for ras expression. The expression of Xenopus ras at a high level during oogenesis and early development suggests a major function of this gene both in meiosis and in mitosis events during embryonic development.