MITOCHONDRIAL BIOGENESIS IN EARLY MOUSE EMBRYOS - EXPRESSION OF THE MESSENGER-RNAS FOR SUBUNIT-IV, SUBUNIT-VB, AND SUBUNIT-VIIC OF CYTOCHROME-C-OXIDASE AND SUBUNIT-9 (P1) OF H-ATP SYNTHASE()

The mouse egg contains about 90,000 mitochondria which undergo a build up of mitochondrial cristae and increase in respiratory activity durin g cleavage. The mitochondrial DNA does not replicate during preimplant ation development but is transcribed actively from the two-cell stage onward (Piko and Taylor, 1987: Dev Biol 123:364-374). To gain further insight into mitochondrial biogenesis, we have now determined the stea dy state amounts of the mRNAs for the cytochrome c oxidase (COX) subun its IV, Vb and VIIc and the H+-ATPase subunit 9 (P1) (all encoded by n uclear genes) in slot hybridization experiments of total RNA from oocy tes and early embryos. All four mRNAs showed a similar developmental p attern of prevalence, characterized by a steady decline in mRNA copy n umbers from the late growth-phase oocyte through the two-cell embryo, and an about 30-fold rise during cleavage through the blastocyst stage . However, the ATPase subunit 9 (P1) mRNA was about three times more p revalent in cleavage-stage embryos than the COX mRNAs. A similar patte rn was obtained previously for the mitochondrial-encoded COX I and II mRNAs, but the latter accumulate at a 30-50-fold excess over the nucle ar-encoded COX subunit mRNAs during the cleavage stages. The results s uggest a coordinated activation and transcription of the mitochondrial and nuclear genes for the components of the respiratory apparatus beg inning with the two-cell stage. It is estimated that new respiratory c hains are produced at a rate of 50-100 chains hr(-1)/mitochondrion in the early blastocyst, accounting for 3.5-7% of the total protein synth etic activity at this stage. This rapid buildup of the mitochondrial o xidative phosphorylation system appears to be mostly preparatory for p ostimplantation development. (C) 1995 Wiley-Liss, Inc.