In a recent study we showed that the Mu1 and Mu1.7 elements of Robertson's Mutator lines can be detected as extrachromosomal DNA circles in active Mu lines. (V. Sundaresan and M. Freeling, PNAS 84:4924, 1987). We have now found that several other Mu1 homologous elements appear also to be present as extrachromosomal DNA circles based on hybridization to isolated circular fractions in Southern blots. A library of the extrachromosomal circular DNA from an active Mu line has been constructed and three clones that exhibited Mu1 homology have been characterized. They were found to contain inserts of 2 kb, 1.4 kb and 0.9 kb, respectively, that exist as multi-copy sequences in the chromosome. The structures of the cloned sequences suggest that they carry the inverted repeat termini of Mu1 with internal sequences non-homologous to Mu1. Elements with this structure resemble the "endogenous Mu" elements that have been observed in both Mu and normal lines (C. Rivin, et al.; MNL 61:54, 1987).
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