DIFFERENTIAL STABILITY OF XENOPUS C-MYC RNA DURING OOGENESIS IN AXOLOTL - INVOLVEMENT OF THE 3'-UNTRANSLATED REGION IN-VIVO

We have used the axolotl oocyte (Ambystoma mexicanum Shaw) to study th e stability of exogenously injected Xenopus RNAs. Three different cell ular developmental stages have been analysed: (1) the growing oocyte ( stage III-IV of vitellogenesis), (2) the full-grown oocyte at the end of vitellogenesis (stage VI) and (3) the progesterone-matured stage VI oocyte. Three exogenous RNAs have been synthesized in vitro from a c- myc Xenopus cDNA clone. One transcript is 2.3 kb long (full length), t he second is 1.5 kb long, with most of the 3' untranslated region (3'U TR) removed, and the third corresponds to the 3'UTR (0.8 kb). After in jection or coinjection of these exogenous Xenopus RNAs into axolotl oo cytes, the stability of the molecules was studied after 5 min, 6 h and 21 h by extraction of total RNA and Northern blot analysis. Results s how a difference in Xenopus RNA stability during axolotl oogenesis. In growing oocytes, the three synthetic transcripts are gradually degrad ed. The absence of the 3'UTR is not therefore sufficient to stabilize the transcript during early oogenesis. No degradation is observed in f ull-grown oocytes, suggesting the existence of stabilizing factors at the end of oogenesis. When stage VI oocytes are induced to mature by p rogesterone, only the 2.3 and 1.5 kb Xenopus RNAs disappear. This sugg ests a role for germinal vesicle breakdown in this degradation process as well as the existence of a factor present in the nucleus and invol ved in the specific destabilization of these RNAs after oocyte maturat ion. This degradation might implicate several destabilizing sequences localized in the coding or in the 3'UTR of the c-myc gene. In contrast , the 0.8 kb transcript (3'UTR) is not degraded during this period and remains very stable. Therefore, degradation appears distinct from one transcript to another and from one region to another within the same molecule. During maturation, the behaviour of the 2.3 and 1.5 kb trans cripts is different when coinjected with the 3'UTR, suggesting a role in trans of this untranslated molecule in c-myc stability. Our approac h allows us to analyse the role of the coding and 3'UTR regions of the c-myc RNA in the control of mRNA degradation in vivo.