Saccharomyces cerevisiae 2-mum DNA. An analysis of the monomer and its multimers by electron microscopy

The non-tandem inverted duplication in the 2-mum DNA of Saccharomyces cerevisiae has a length of 0.19 mum and is located asymmetrically along the molecule. The majority of the dumb-bell structures that are formed upon denaturation and self-annealing of the 2-mum monomer consists of the renatured inverted duplication sequences as double stranded stem and two single stranded loops of 0.67mum+/-0.06 mum (S-loop) and 0.86 mum+/-0.05 mum (L-loop) length. Two additional size classes which comprised 5-10% of the measured molecules had contour lengths of around 1.7 mum and 2.1 mum. 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-mum and 6-mum circular molecules present in the yeast supercoiled DNA fraction are shown to be dimers and trimers of 2-mum monomers, but often with inverted loop segments most probably due to intramolecular recombination between sequences of the inverted duplication.