Mechanism for circularization of linear DNAs: circular parvovirus MVM DNA is formed by a "noose" sliding in a "lasso"-like DNA structure.

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Abstract
During an EM survey with the aim of identifying the parvovirus MVM transcription template, previously unidentified structures of MVM DNA were observed in lysates of virus-infected [mouse fibroblast] cells. These included double-stranded lasso-like structures and relaxed circles. Both structures were of unit length MVM DNA, indicating that they were not intermediates formed during replication; they each represented about 5% of the total nuclear MVM DNA. The proportion of these structures was unchanged after digestion with sodium dodecyl sulfate/Pronase and RNase and after mild denaturation treatment. Cleavage of the lasso structures with EcoRI restriction endonuclease indicated that the noose part of the lasso structure is located on the 5'' side of the genomic single-stranded MVM DNA. A model is presented for the molecular nature of the circularization process of MVM DNA in which the lasso structures are identified as intermediates during circle formation. This model proposes a mechanism for circularization of linear DNA.