Heterologous transposition in Ustilago maydis

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Abstract
The phytopathogenic basidiomycete Ustilago maydis has become a model system for the analysis of plant-pathogen interactions. The genome sequence of this organism will soon be available, increasing the need for techniques to analyse gene function on a broad basis. We describe a heterologous transposition system for U. maydis that is based on the Caenorhabditis transposon Tc1, which is known to function independently of host factors and to be active in evolutionarily distant species. We have established a nitrate reductase based two-component counterselection system to screen for Tc1 transposition. The element was shown to be functional and transposed to several different locations in the genome of U. maydis. The insertion pattern observed was consistent with the proposed general mechanism of Tc1/mariner integration and constitutes a proof of principle for the first heterologous transposition system in a basidiomycete species. By mapping the insertion site context to known genomic sequences, Tc1 insertion events were shown to occur on different chromosomes, but exhibit a preference for non-coding regions. Only 20% of the insertions were found in putative open reading frames. The establishment of this system will permit efficient gene tagging in U. maydis and possibly also in other fungi.