CIRCULAR DNA OF A YEAST EPISOME WITH 2 INVERTED REPEATS - STRUCTURAL-ANALYSIS BY A RESTRICTION ENZYME AND ELECTRON-MICROSCOPY

Small circular DNA molecules from genetically characterized clones of Saccharomyces cerevisiae were studied by restriction endonuclease analysis and EM. The circular monomers (6000 bases) are shown to contain 2 inverted repeats of the same sequence (600 bases) situated opposite each other along the perimeter. Four endonuclease EcoRI fragments are obtained in 1:1:1:1 stoichiometry, and their sum gives a length of about 12,000 bases. The 2 large fragments and the 2 small ones differ from each other by 200 bases. A model for the structure of the monomer molecule is proposed. Two classes of monomers can be generated by intramolecular recombinations within inverted repeats; they differ by the relative orientation of nonrepeated segments. The structure of dimers as predicted by the model is verified by self-renaturation of single-stranded circles. Inverted repeats in circular molecules may be related to the insertion release faculty of II episome in the chromosomes.