Post-transcriptional control of c-myc RNA during early development analyzed in vivo with a Xenopus-axolotl heterologous system

We have set up a heterologous in vivo system to study gene regulation at the post-transcriptional level during early development. This system uses two amphibian species, Xenopus laevis and Ambystoma mexicanum (axolotl), the development of which is three to four times slower than that of X. laevis. The stability of three different synthetic X. laevis c-myc transcripts was followed after injection into fertilized axolotl eggs. One transcript is 2.2 kilobases (kb) long (full-length). The second is 1.5-kb long with most of the 3' untranslated region (3'UTR) removed, and the third corresponds to the 3'UTR (0.7-kb). The behavior of the endogenous axolotl c-myc RNA was compared with the exogenous injected c-myc transcripts. Our results show the existence of several developmental timers controlling degradation of the c-myc molecules. The first is activated at oocyte maturation and affects both the endogenous and exogenous (2.2- and 1.5-kb) transcripts containing the coding regions. A second timer could be linked to the number of cell divisions since fertilization (6th-7th cleavages) and involves the endogenous c-myc RNAs. Another timer could involve the c-myc mRNA molecule itself, because when injected into axolotl eggs, the half-life of the 2.2-kb X. laevis transcript appears to be independent of the axolotl context. After injection into axolotl fertilized eggs, the behavior of this X. laevis full-length c-myc molecule reveals an unexpected increase in the intensity of its autoradiographic signals. This increase occurs independently of events linked to mid-blastula transition and preliminary investigations are discussed.