The arrangement of ribosomal RNA genes in Schistosoma mansoni. Identification of polymorphic structural variants.

The two large ribsomal RNA subunits of Schistosoma mansoni are encoded within a 10 000-base sequence, which is tandemly repeated in the schistosome genome. Restriction endonuclease digestion with Bam HI cuts the rRNA gene into three fragments, which have been clones separately in pBR322 and used to constract a physical map of the gene. The sequence encoding the smaller rRNA subunit is about 2000 bases in length and is situated on the 5' sie of the sequence encoding the larger subunit, which is about 4000 bases. Approximately 4000 bases of the rRNA gene are spacer and do not code for mature rRNA. There are approximately 100 copies of the rRNA gene per haploid genome of which about 10% exhibit length heterogeneity, as judged by the hybridization of the rDNA plasmids to restriction endonuclease digests of genomic DNA. These structural variants appear to contain additional DNA sequences at more than one site within the gene and are polymorphic within the species S. mansoni.