Abstract
Mitochondrial gene expression has been analysed during embryonic development of Xenopus laevis; the relative amounts of 12S and 16S ribosomal RNAs and of most mitochondrial messenger RNAs were determined by slot-blot and Northern-blot hybridization experiments with specific mitochondrial DNA probes. The rRNA content per embryo remained constant during early development, confirming earlier results of Chase and Dawid (1972, Dev. Biol., 27, 504-518.); on the contrary, all mRNAs decreased abruptly after fertilization within a few hours (by a factor of 5-10), remained at a very low level up to the late neurula stages and increased again during organogenesis. Since the mitochondrial DNA content does not vary during this period molecular analyses as well as biological observations suggest that the mitochondrial genome is completely inactivated at the beginning of embryonic development. The amounts of rRNAs and mRNAs evolve therefore as a function of time only according to their half-lives. Mitochondrial RNA accumulation resumes subsequently with a high rate when the general transcription in the embryo is starting again, and this occurs before the resumption of DNA replication in the organelle. It appears that the Xenopus embryonic development represents a quite clear example of regulation of the mitochondrial expression at the level of transcription.