Isolation and mapping of the rRNA genes in the macronucleus of Oxytricha fallax

The DNA in the macronucleus of the protozoan Oxytricha, unlike like that of typical eukaryotes, exists as short, gene-sized molecules. Within the macronucleus the rRNA genes are contained in molecules 7,380 nucleotide pairs in length. This rDNA has been substantially purified by selective denaturation of non-ribosomal DNA followed by S1 nuclease digestion. Results from restriction nuclease digestion and rRNA:DNA hybridization who that the rDNA is a linear, non-palindromic molecule which contains one gene each for the 19s and 25s rRNAs. A total of less than 600 base pairs of DNA lies between the 19s and 25s genes or at the 3' end of the 25s gene. The non-coding portion of the ribosomal DNA is almost entirely limited to an approximately 1,400 base pair region at the 5' end of the molecule.