Cloning and characterization of the ribosomal genes of the sea‐urchin Paracentrotus lividus

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Abstract
A Paracentrotus lividus genomic library was constructed using sperm DNA prepared from a single animal. The DNA was fragmented by partial digestion with DNase II, sized on a preparative agarose gel and inserted in the PstI site of pBR 322 by the dG · dC tailing method. Recombinant plasmids containing ribosomal DNA were isolated, a restriction map of the gene was determined and the 18S and 26S transcribed sequences were located by S1 protection mapping.The organization of the ribosomal genes in genomic DNA of individual animals and of a pool of animals was studied by blot‐hybridization of the restriction fragments, using as probes nick‐translated 32P‐labelled cloned ribosomal DNA fragments or 18S and 26S sea‐urchin ribosomal RNA. The repeat length of the ribosomal unit was about 10.5 × 103 bases. A comparison of the restriction patterns of DNA from different animals showed a marked sequence heterogeneity in the spacer region of these genes. Variations of about 200 base pairs were detectable in the length of the spacer of some individuals.