Synthesis of mitochondrial RNA by full-grown and maturing oocytes of Rana pipiens and Xenopus laevis

Abstract

Full-grown oocytes of Rana pipiens and Xenopus laevis are found to synthesize mitochondrial RNA both before and during progesterone-induced maturation in vitro. Gel electrophoresis of labeled mitochondrial (mt) RNA reveals that maturing oocytes synthesize “large” and “small” mitochondrial rRNA's, 4 S RNA and heterodisperse RNA. Prior to germina vesicle breakdown in Xenopus stage 6 oocytes, mtRNA synthesis accounts for less than 2% of total RNA production. After nuclear dissolution, substantially more [ 3H]GTP incorporation can be considered mitochondrial in origin, due primarily to a dramatic reduction in nuclear-directed RNA synthesis. Levels of mtRNA-associated radioactivity after 5–6-hr pulses are essentially the same at all times before and during maturation, suggesting a constant rate of mtRNA synthesis throughout maturation. Actual rates of mtRNA synthesis by the Xenopus stage 6 oocyte in vitro have been estimated following analysis of the kinetics of [ 3H]GTP uptake into the mitochondrial acid-soluble pool and incorporation into purified mtRNA. The minimum rates of total mtRNA and mitochondrial rRNA synthesis are calculated to be 21–45 and 6–13 pg/hr/oocyte, respectively. At least a portion of the “DNA-like RNA” reported to be synthesized by the Xenopus oocyte during in vivo ovulation, can be attributed to transcription of mitochondrial rather than nuclear DNA.