Xenopus myc proto-oncogene during development: expression as a stable maternal mRNA uncoupled from cell division

A Xenopus cDNA clone highly homologous to the proto-oncogene c-myc has been isolated and used to derive a homologous probe to study myc expression during embryonic development. Myc RNA is identified as a member of the class of maternal mRNAs expressed before fertilisation. It is highly accumulated from early oogenesis and an unfertilised egg contains 8 pg, about 10(5)-fold the myc content of proliferative somatic cells. After fertilisation a post-transcriptional regulation of the gene is induced and the accumulated myc RNA is degraded (t1/2 = 4 h 20 min) to reach a level at gastrula of 10 transcripts per cell; a value maintained during subsequent embryonic development. The Xenopus myc protein has also been identified by both myc-specific antibodies and hybrid selection experiments. Translation in vitro of Xenopus myc RNA shows that it encodes a 62-kd protein which is also recognised by myc antibodies in oocyte extracts. This protein is accumulated in late oogenesis. The results indicate an unusual uncoupling of myc expression and cell proliferation linked to a stabilisation of the RNA product.