Amphibian protein synthesis

Achen, M. G., et al. (1992). Protein synthesis at the blood-brain barrier. The major protein secreted by amphibian choroid plexus is a lipocalin. J. Biol. Chem. 267, 23170-4. 

Benbassat J (1974) The transition from tadpole to frog haemoglobin during natural amphibian metamorphosis. I. Protein synthesis by peripheral blood cells in vitro. J Cell Sci 15 347-357... 

Each mitochondrion contains several molecules of DNA (mtDNA), usually in a closed, circular form, as well as the ribosomes, tRNA molecules, and enzymes needed for protein synthesis.1 23 26 With rare exceptions almost all of the mitochondrial DNA in a human cell is inherited from the mother.6 263 The size of the DNA circles varies from 16-19 kb in animals27 to over 200 kb in many higher plants. Complete sequences of many mitochondrial DNAs are known.28 283 Among these are the 16,569 bp human mtDNA,29 the 16,338 bp bovine mtDNA, the 16,896 bp mtDNA of the wallaroo Macropus robustus,30 and the 17,533 bp mtDNA of the amphibian Xenopus laevis.31 32 The sea urchin Paracentotus lividus has a smaller 15,697 bp genome. However, the order of the genes in this and other invertebrate mtDNA is different from that in mammalian mitochondria.33 Protozoal mtDNAs vary in size from 5900 bp for the... 

We mentioned studies of Chase and Dawid (1972) which clearly showed that during early embryonic development in Xenapus laevis mt DNA is not replicated. These studies also established that the mitochondrial genome is apparently not transcribed during these early stages of development in amphibians and that there is no protein synthesis at the mitoribosomes. No net increase in mitochondrial protein or in cytochrome oxidase activity was found (cf. Table VI). 

Interesting changes are also observed in the structure of the nucleoli in the oocytes of amphibians. A large number of nucleoli appear during the active elaboration of proteins. However, when the oocyte reaches maturity, or later when it divides (physiological stages that are accompanied by relatively little protein synthesis), the nucleoli become small and even disappear. 

In addition, it has been demonstrated that RNA synthesis in the maturing oocytes of mammals is under the control of the surrounding follicule cells. RNA synthesis in various stages of oocyte maturation was changed (Fig. 51) and was terminated in large oocytes. It was found that the anthrum fluid, which is produced by follicular cells, contains compounds that inhibit DNA dependent RNA-polymer-ase activity in vitro (Moore et al., 1974). A proteinous factor was extracted from embryonic vesicles of amphibian oocytes. This factor inhibits transcription of the ribosomal genes (Crippa, 1970 Crippa et al., 1972). 

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