Specific expression of the Fusarium transposon Fot1 and effects on target gene transcription.

The Fot1 transposon is active in some strains of the plant pathogenic fungus Fusarium oxysporum. In a high-copy-number strain that contains autonomous elements, we have detected a transcript of 1.7 kb hybridizing to Fot1 in very low amounts. Mapping the 3' and 5' termini of this transcript confirms that it corresponds to a Fot1-specific transcript. In this strain, five independent mutants of the transgene (niaD) encoding nitrate reductase have arisen by insertion of Fot1 into the third intron. The analysis of the effect of Fot1 insertion in these mutants shows that, depending on the orientation of Fot1 relative to niaD, different truncated chimeric niaD-Fot1 transcripts are produced. Mapping the 5' and 3' ends of these transcripts reveals (i) premature polyadenylation at sites present in the 5' and 3' untranslated regions of Fot1, and (ii) initiation of some transcripts in the 3' part of the niaD gene at sites located immediately downstream of the Fot1 insertion. Thus, a novel promoter, associated with the end of Fot1, directs transcriptional activity outwards from the element into the coding sequence of the niaD gene. These effects demonstrate that Fot1 insertion provides an additional general mechanism controlling fungal gene expression.