Cloning and characterization of the ribosomal genes of the sea-urchin <em>Paracentrotus lividus</em>.

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 Pst I site of pBR 322 by the dG &#183; 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 S l 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 &#215; 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. [ABSTRACT FROM AUTHOR]