Hypoxic depression of mitochondrial mRNA levels in Hela cell

Steady-state levels of 12 S and 16 S mitochondrial (mt) rRNAs and four mRNAs (NADH dehydrogenase subunits 3 and 4/4L, cytochrome c oxidase subunit III, H⁺-ATPase subunits 6/6L) were estimated by hybridization of cellular RNA with cloned human mt DNA fragments during hypoxia of HeLa cells. When the partial pressure of oxygen (pO₂) was shifted from 135 to 15 Torr, the level of all mRNAs coordinately decreased more than 95% in 48 h, while that of rRNAs remained virtually unchanged. mRNA levels recovered within 4-6 h of reexposure to normoxia. The depression was observed at pO₂ less than 40 Torr, the physiological pO₂ in peripheral tissues. During these transitions, the growth rate of HeLa cells and the copy number of mt DNA per cell remained unchanged. The degradation rate of mt mRNAs in the presence of cordycepin was not affected by pO₂. In contrast to the in vivo results, the potential activity of transcription in isolated mitochondria assayed under optimum conditions was not affected by previous hypoxic exposure of the cells. These observations provide evidence for the existence of a new mechanism controlling mitochondrial gene transcription.