Circular DNA of a yeast episome with two inverted repeats: structural analysis by a restriction enzyme and electron microscopy.

Small circular DNA molecules from genetically characterized clones of Saccharomyces cerevisiae have been studied by restriction endonuclease analysis and electron microscopy. The circular monomers (6000 bases) are shown to contain two 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 two large fragments and the two small ones differ from each other by 200 bases. We propose a model for the structure of the monomer molecule. 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.