Mitochondrial gene expression is regulated at the level of transcription during early embryogenesis of Xenopus laevis

Mitochondrial transcription in the early Xenopus laevis embryo resumes several hours before active mtDNA replication, effectively decoupling mtDNA transcription and replication. This developmental feature makes Xenopus embryogenesis an appealing model system to investigate the regulation of mitochondrial transcription. Studies reported here refine our understanding of the timing, magnitude, and mechanism of this transcriptional induction event. Northern analyses of six mitochondrial mRNAs (normalized to mtDNA) reveal that transcript levels remain basal between fertilization and gastrulation and then undergo a coordinate induction, culminating in a 20-28-fold increase over egg levels by 48 h of development. Measurement of mitochondrial run-on transcription rates demonstrates a good correlation between transcription rates and transcript levels, showing that transcription itself is the primary determinant of transcript abundance. Experimental increases in mitochondrial ATP and energy charge also correlate with patterns of transcript levels and transcription rates, suggesting that developmental changes in the biochemical composition of the mitochondrial matrix could be regulating transcriptional activity. Consistent with this idea, transcriptional run-on rates in mitochondria of early embryos can be stimulated by the addition of tricarboxylic acid cycle intermediates to the run on reaction. However, mitochondria of later stages do not show this response to the addition of metabolite. In combination, these data suggest that mitochondrial transcription is under metabolic regulation during early Xenopus embryogenesis.