Perhaps the most fundamental question that must be answered for any transposable element system is: "What encodes the autonomous (transposase) function and how is it transmitted?" That is the primary question we address for the Mu transposon system in maize. Five different approaches are proposed. 1) A newly discovered Mu transposon is structurally consistent with autonomy. The element will be added using T-DNA to a special tester line of tobacco plantlets: If the transposon is autonomous it will specify "excisase" that will revert a tobacco gene, leading to a kanomycin-resistent tobacco. A similar experiment will be done using maize cells. 2) We recently discovered that some Mu transposons have circular forms. These will be microinjected into the tester tobacco line and into maize. 3) New Mu transposons will be characterized. 4) We will use a series of genetic crosses to evaluate hypotheses for Mu copy number control, and will also obtain mutants affecting Mu excision by selecting rare "phase change" phenotypes. 5) We also propose to identify the transposase directly by looking for specific protein-Mu binding. A secondary aim involves characterization of Mu excision products. The information sought in these proposed experiments is not only essential for our understanding of this truly unique transposon system, but is essential if Mu is to be useful for biotechnology.
