Comparative analysis of nuclear ribosomal DNA from the moon jelly Aurelia sp.1 (Cnidaria: Scyphozoa) with characterizations of the 18S, 28S genes, and the intergenic spacer (IGS)

Nuclear ribosomal DNAs (rDNA) constitute a multi-gene family with tandemly arranged units linked by an intergenic spacer (IGS). Here we present the complete DNA sequence (7,731 bp) of a single repeat unit of an rDNA sequence from the moon jelly Aurelia sp.1 (Cnidaria: Scypozoa). The tandemly repeated rDNA units consisted of coding and non-coding regions, whose arrangement was 18S rDNA (1,814 bp, 46.2% of GC content)-internal transcribed spacer 1 (ITS1: 272 bp, 39.7%)-5.8S rDNA (158 bp; 50.7%)-ITS2 (278 bp, 51.4%)-28S rDNA (3,606 bp, 49.7%)-IGS (1,603 bp, 45.6%). GC composition in the single unit of rDNA was 47.8%. None of the 5S rDNA was found in the repeat units. Putative structures of a termination transcription signal (poly(T) tract) and promoter-like bi-repeats within the non-coding region were also identified. A block of minisatellites with five repeats was detected within the IGS. Comparative analyses of parsimony and dot plots showed that the IGS was highly informative. The sequence revealed here was the first completion of rDNA from the phylum Cnidaria, using as a model of rDNA for making molecular comparisons of jellyfish members.