Regulation of mutator activities in maize.

We discuss the properties of the Mutator (Mu) transposable element family of maize. We report the cloning of bz2-mu1, a mutable allele containing a 1.4-kb Mu element, using a combination of transposon tagging and tests for differential hybridization to northern and Southern blots. We report the sequence of this allele and the Mu element insertion, and propose a model for the structure of the Bz2 locus. We discuss the relationship between increased DNA modification of Mu elements and loss of somatic instability at bz2-mu1. To further explore this aspect of regulation of Mutator, we have used gene-specific probes to determine the level of modification at this locus in active and inactive Mutator lines. We have also utilized CsCl density gradients to estimate the overall level of DNA modification in active and inactive lines; we find that Mu elements in active lines are hypomethylated relative to other maize nuclear DNAs examined, and that in inactive lines the level of modification in Mu elements is similar to the genome as a whole. Utilizing gamma-irradiation, we have demonstrated that inactive lines can be reactivated; this reactivation is first noted as restitution of the spotted kernel phenotype characteristic of bz2-mu1 in active Mutator lines. Hybridization analysis of DNA from reactivated plants demonstrates that the Mu elements in general, and specifically the Mu element at bz2-mu1, have the lower level of DNA modification characteristic of active lines. These results are discussed in terms of the role and timing of DNA modification in regulating Mutator activities.