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In the present paper the in vivo incorporation of 3H-guanosine into mitochondrial RNA and of 35S-methionine into mitochondrial ribosomal proteins of Paracentrotus lividus embryos have been studied. Mitochondrial rRNA and low molecular weight RNA are newly synthesized in vivo by Paracentrotus lividus embryos as early as at 2-cell stage. Labeled RNA but not labeled proteins have been extracted from the mitochondrial ribosomes of the Paracentrotus lividus embryos at 32-cell stage.

In order to study the gene organization and sequence of the mt DNA from sea urchin Paracentrotus lividus, we have cloned the two Bam HI fragments of 12 Kb and 4 Kb in the plasmid vectors pAT 153 and pBR 322 respectively. The cloning has been tested by hybridizing the DNA extracted from the recombinant plasmids with sea urchin mt DNA and by comparing the restriction patterns of the cloned fragments with those shown by fragment isolated from sea urchin mt DNA. This has allowed the determination of the size of sea urchin mt DNA that results to be about 15.000 base pairs, similar to that already reported for other animal mt DNA.

In vivo labeling of mitochondrial DNA using 3H-methyl-thymidine suggests that in the sea urchin Paracentrotus lividus also no bulk replication of mitochondrial DNA takes place after fertilization at least until pluteus stage.