Transposition of the autonomous Fot1 element in the filamentous fungus Fusarium oxysporum

Autonomous mobility of different copies of the Foil element was determined for several strains of the fungal plant pathogen Fusarium oxysporum to deve lop a transposon tagging system. Two Fot1 copies inserted into the third in tron of the nitrate reductase structural gene (niaD) were separately introd uced into two genetic backgrounds devoid of endogenous Foil elements. Mobil ity of these copies was observed through a phenotypic assay for excision ba sed on the restoration of nitrate reductase activity. Inactivation of the F oil transposase open reading frame (frameshift, deletion, or disruption) pr evented excision in strains free of Foil elements. Molecular analysis of th e Nia(+) revertant strains showed that the Foil element reintegrated freque ntly into new genomic sites after-excision and that it can transpose from t he introduced niaD gene into a different chromosome. Sequence analysis of s everal Fot1 excision sites revealed the so-called footprint left by this tr ansposable element. Three reinserted Fot1 elements were cloned and the DNA sequences Ranking the transposon were determined using inverse polymerase c hain reaction. In all cases, the transposon was inserted into a TA dinucleo tide and created the characteristic TA target site duplication. The availab ility of autonomous Foil copies will new permit the development of an effic ient two-component transposon tagging system comprising a tr trans-activato r element supplying transposase and a cis-responsive marked element.