Characterization of the Gyrodactylus salaris Malmberg, 1957 (Platyhelminthes: Monogenea) ribosomal intergenic spacer (IGS) DNA

The intergenic spacer of the ribosomal RNA gene array from the monogenean Gyrodactylus salaris was isolated using PCR amplification, PCR products were cloned and sequenced. Three different fragments of 0.63, 1.0 and 2.62 kb, were consistently obtained. These showed homology at the 5' and 3' termini but differed in their overall size and intervening sequence. The 5' end showed homology to various 28S ribosomal RNA gene sequences suggesting that this represented the 3' terminus of the G. salaris 28S ribosomal RNA gene. A number of features common to other eukaryotic intergenic spacers were found in the longest sequence, including A + T rich sequences, palindromic sequences and tandemly repeated elements. Two regions of 23 bp sequences arranged in non-identical tandem repeats were identified. There were 9 repeats in both regions, separated by 81 bp of non-repetitive sequence. The repeat units from the two regions shared some similarity at their 3' ends. The G. salaris intergenic spacer sequence was examined for sequence motifs involved in the transcription of the ribosomal RNA genes in other species. Several regions with homology to transcription start sites were identified.