Stable non-mutator stocks of maize have sequences homologous to the Mu1 transposable element

Mutator stocks of maize produce mutants at many loci at rates 20- to 50-fold above spontaneous levels. Current evidence suggests that this high mutation rate is mediated by an active transposable element system, Mu. Members of this transposable element family are found in approximately 10-60 copies in Mutator stocks. We report here an initial characterization of previously undetected sequences homologous to Mu elements in eight non-Mutator inbred lines and varieties of maize that have a normal low mutation rate. All stocks have approximately 40 copies of sequences homologous only to the terminal repeat and show weak homology to an internal probe. In addition, several of the stocks contain an intact Mu element. One intact Mu element and two terminal-specific clones have been isolated from one non-Mutator line, B37. The cloned sequences have been used to demonstrate that in genomic DNA the intact element, termed Mu1.4B37, is modified, such that restriction sites in its termini are not accessible to cleavage by the HinfI restriction enzyme. This modification is similar to that observed in Mutator lines that have lost activity. We hypothesize that the DNA modification of the Mu-like element may contribute to the lack of Mutator activity in B37.