Epigenetic Regulation of the Mutator System of Transposons

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Transposable elements, or transposons, are short stretches of DNA that can move from place to place within genomes. Often they move into, and thus mutagenize, other genes. Because of this propensity, transposons have become remarkably useful mutagens, as they both mutate and "tag" genes, allowing for their rapid recovery, particularly in large, unsequenced genomes such as maize. In maize, the Mutator transposon system has become the primary means by which genes are mutagenized and cloned. The system is composed of a number of different classes of elements, all of which are regulated by the MuDR class. Mutator (Mu) is remarkably active; elements can duplicate themselves at frequencies exceeding one new insertion per element per generation, resulting in massive increases in mutation frequencies. In order to exploit the systems utility, it is necessary to understand its regulation. The aspect of Mu regulation that is least understood involves epigenetic modification of otherwise active MuDR elements. Epigenetic modification involves changes in gene activity without changes in DNA sequence. In the case of Mutator these changes can result in a heritable loss of activity. This project seeks to understand the nature of epigenetic shifts in Mutator system. The project will exploit the availability of a minimal Mutator line, which, unlike those lines used for mutagenesis, carries a single MuDR element at a known chromosomal position. Mutations and chemical agents that either silence or reactivate this single MuDR element will be used to dissect the molecular events that correspond to these changes in Mutator activity. Using the minimal Mutator system it will also be possible to separate the processes involved in the initiation of silencing from those involved in its maintenance, an open question for a variety of epigenetically regulated phenomena. In addition, the system will be used to examine the effects of a large number of mutations in chromatin remodeling genes in maize that are being developed at the University of Arizona as part of a previously funded project. It is expected that this research will make the Mutator system significantly more useful to the maize community, and that it will illuminate a variety of questions concerning epigenetic silencing, both in maize and in other organisms. Of particular and immediate use to the maize community will be a locus we have identified designated Mu killer (Muk), which can silence otherwise highly active and mutagenic maize Mu lines. Lines carrying the Muk locus will be made available, along with instructions as to their use. In addition, initial results of interactions between mutations affecting chromatin structure and silenced MuDR elements will be made public on the University of Arizona chromatin database(ChromDB: http://www.chromdb.org/) as they become available.