Structural Analysis of Ribosomal RNA Genes from the Hawaiian Sea Urchin Species,Tripneustes gratilla

Ribosomal gene sequences (rDNA) were isolated from the genomic DNA of the Hawaiian sea urchin Tripneustes gratilla by cloning in plasmid and phage vectors. The rRNA coding regions in four clones were localized by hybridizing Southern transfers of endonuclease-digested DNA with ³²P-labeled 26S rRNA and ³²P-labeled 18S rRNA. Three of the rDNA clones were isolated from a library of DNA from a single sea urchin and represent the two major types of rDNA repeats present in that individuals's genome. Both repeat types appear identical within their rRNA coding regions but are dissimilar in an area of the nontranscribed spacer (NTS) adjacent to the 3′ end of the 26S coding region. The cloned rDNA repeats were shorter than their genomic counterparts due to deletions occurring within internally repetitious NTS domains, probably as a result of unequal recombination during phage propagation.