The transposable element Mu-1 has been isolated from maize lines exhibiting Robertson's Mutator activity, characterized by a mutation rate 20-50 fold higher than normal (Bennetzen et al., P.N.A.S. 81:4125, 1984). Mu-1 is 1367 bp in length and contains terminal inverted repeats of 213 and 215 bp and four open reading frames (Barker et al., N.A.R. 12:5955, 1984).
We have isolated 30 Mu-1 homologous clones from a Mutator library. Twenty-six contain elements of the same size class as Mu-1, but four contain Mutator elements which differ from Mu-1 in both size and organization. Mu-L, a 1.7 kbp element, contains 300 bp which are not present in Mu-1. Electron microscopy and hybridization experiments show that the additional 300 bp are not a duplication of sequences found in other Mutator elements, nor are they highly represented in the DNA of non-Mutator plants. The other three novel elements are smaller than Mu-1. Mu-S1 (1.0 kbp) has intact terminal repeats and internal segments which are homologous to Mu-1. Both Mu-L and Mu-S1 contain small deletions and/or restriction site polymorphisms which indicate they could not have arisen from Mu-1 by a single insertion/deletion event. Mu-S2 (800 bp) and Mu-S3 (650 bp) do not have intact terminal repeats and show less homology to Mu-1. We are sequencing the elements to determine their coding capacity and their relationship to Mu-1. The 300 additional base pairs present in Mu-L may have the capacity to code for a protein. Approximately 250 bp of this additional DNA has been subcloned for use as a specific probe with which to follow the segregation and transposition of Mu-L independently of the other elements in the Mutator system.
Loverine P. Taylor and Virginia Walbot
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