Saccharomyces cerevisiae 2-μm DNA

Abstract

The non-tandem inverted duplication in the 2-μm DNA of Saccharomyces cerevisiae has a length of 0.19 μm and is located asymmetrically along the molecule. The majority of the dumb-bell structures that are formed upon denaturation and selfannealing of the 2-μm monomer consists of the renatured inverted duplication sequences as double stranded stem and two single stranded loops of 0.67 μm±0.06 μm (S-loop) and 0.86 μm±0.05 μm (L-loop) length. Two additional size classes which comprised 5–10% of the measured molecules had contour lengths of around 1.7 μm and 2.1 μm. The smaller dumb-bells contained two S-loops and the larger dumb-bells contained two L-loops as was shown by heteroduplex mapping with an HindIII fragment from the L-loop. Two models which assume illegitimate or site specific recombination, are presented to explain the generation of double S-loop and double L-loop molecules. At least part of the 4-μm and 6-μ circular molecules present in the yeast supercoiled DNA fraction are shown to be dimers and trimers of 2-μm monomers, but often with inverted loop segments most probably due to intramolecular recombination between sequences of the inverted duplication.