Cv. Ammini et Ww. Hauswirth, Mitochondrial gene expression is regulated at the level of transcription during early embryogenesis of Xenopus laevis, J BIOL CHEM, 274(10), 1999, pp. 6265-6271
Mitochondrial transcription in the early Xenopus laevis embryo resumes seve
ral hours before active mtDNA replication, effectively decoupling mtDNA tra
nscription and replication. This developmental feature makes Xenopus embryo
genesis an appealing model system to investigate the regulation of mitochon
drial transcription. Studies reported here refine our understanding of the
timing, magnitude, and mechanism of this transcriptional induction event. N
orthern analyses of six mitochondrial mRNAs (normalized to mtDNA) reveal th
at transcript levels remain basal between fertilization and gastrulation an
d then undergo a coordinate induction, culminating in a 20-28-fold increase
over egg levels by 48 h of development. Measurement of mitochondrial run-o
n transcription rates demonstrates a good correlation between transcription
rates and transcript levels, showing that transcription itself is the prim
ary determinant of transcript abundance. Experimental increases in mitochon
drial ATP and energy charge also correlate with patterns of transcript leve
ls and transcription rates, suggesting that developmental changes in the bi
ochemical composition of the mitochondrial matrix could be regulating trans
criptional activity. Consistent with this idea, transcriptional run-on rate
s in mitochondria of early embryos can be stimulated by the addition of tri
carboxylic acid cycle intermediates to the run on reaction. However, mitoch
ondria of later stages do not show this response to the addition of metabol
ite. In combination, these data suggest that mitochondrial transcription is
under metabolic regulation during early Xenopus embryogenesis.