Specific Properties of the End Groups of rDNA from Tetrahymena

Abstract

In some eukaryotic cells the DNA coding for ribosomal RNA exists as extrachromosomal molecules. This has been demonstrated for oocytes of various animals (1) and for some primitive organisms like Tetrahymena and Physarum. From the latter two organisms this kind of discrete DNA molecules (rDNA) has been isolated and characterized. They have in both cases been found to contain an axis of two-fold rotational symmetry; i.e., the individual strands are self complementary (2,3,4). DNA molecules with this special arrangement of base sequence is often referred to as palindromic molecules. Heating and rapid cooling of solutions of palindromic DNA molecules results in formation of fold-back or smap-back forms of hairpin-like structures with molecular weights which are half of that of the native molecules. The special arrangement of bases in palindromic molecules also means that cleavage with site specific endonucleases results in odd numbers of fragments. The rDNA of Tetrahymena strain BVII which has a molecular weight of about 13.2 x 10⁶ has thus been found to give rise to three fragments by treatment with restriction enzyme EcoRI, two identical fragments (molecular weight 1.6 x 10⁶) from the ends of the molecule and one fragment containing the center part of the molecule (molecular weight 10.0 x 10⁶) (3). The snap-back form of this rDNA molecule would be expected to give rise to one each of these two fragments by treatment with EcoRI, i.e., the center fragment in hairpin form (molecular weight 5.0 x 10⁶) and the end group fragment.