Mitochondrial Gene Expression Is Regulated at the Level of Transcription during Early Embryogenesis of Xenopus laevis

Abstract

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.

Endotoxin-Stimulated Nitric Oxide Production Inhibits Expression of Cytochrome c Oxidase in ANA-1 Murine Macrophages

“…Once initiated, the strands are transcribed as single polycistronic precursor RNAs. Increased mitochondrial transcription has been found in rapidly dividing immortal cells requiring elevated respiratory function and anoxia-induced quiescence in Artemia franciscana embryos and in early Xenopus embryogenesis (11)(12)(13)18). However, differential rates of transcription have not been identified among individual mitochondrial polypeptide mRNAs, indicating that differential mRNA stability accounts for varying levels of mitochondrial mRNA levels.…”

“…Mitochondria run-on assays were performed as previously described (11,12). Macrophage mitochondria were prepared in lysis buffer (10 mM Tris-Cl (pH 7.4), 10 mM NaCl, 3 mM MgCl 2 , and 0.5% Nonidet P-40) and pelleted at 500 ϫ g. The mitochondria (2 ϫ 10 7 ) were resuspended in 100 l glycerol buffer, then 150 Ci [␣-32 P]UTP (800 Ci/mmol) in 100 l 10 mM Tris-Cl (pH 8.0), 5 mM DTT, 5 mM MgCl 2 , 300 mM KCl, and 1 mM each of ATP, CTP, and GTP for 30 min at 30°C were added.…”

Tight Control of Respiration by NADH Dehydrogenase ND5 Subunit Gene Expression in Mouse Mitochondria

“…While the large excess of both mitochondrial DNA (mtDNA) and its transcriptional activity could, in principle, allow a rapid adaptation to increased respiratory and ATP synthesis demands, it is intriguing that, in some developmental and physiological situations, an increased level of mitochondrial gene expression is frequently accompanied, and possibly determined, by an increase in the level of mtDNA (9,49,50). Furthermore, there is well-documented evidence of transcriptional regulation of mitochondrial gene expression in rat liver mitochondria by thyroid hormones (16) and during early embryogenesis in Xenopus laevis (1).…”

Response characteristics of the mitochondrial DNA genome in developmental health and disease

“…Nuclear transplantation experiments in Xenopus embryos showed that replication of mtDNA is suppressed by an undefined nuclear factor until the heartbeat stage of organogenesis (Chase and Dawid, 1972;Rinaldi and Giudice, 1985). Once this negative control is lifted, the expression of nuclear-encoded subunits of respiratory enzymes is regulated in a coordinate fashion with the expression of mtDNA coded subunits, mitochondrial ATP, and energy charge (Ammini and Hauswirth, 1999).…”