The degenerate DNA transposon Pat and repeat-induced point mutation (RIP) in Podospora anserina

A degenerate DNA transposon, Par, was identified in the genomes of various wild-type strains of the filamentous fungus Podospora anser inn. In these s trains, the number (approximately 20-25 copies per genome) and location of Pat sequences appear to be conserved. Two copies of Pat, one complete and o ne partial, were cloned and characterized. The sequence of the complete ele ment is 1856 bp long and contains imperfect inverted terminal repeats (ITRs ) of 53 bp. The target site duplication comprises the sequence TA. The amin o acid sequence derived from one reading frame of Pat shows significant hom ology to members of the Fot1 family of transposons. However, this reading f rame is interrupted by numerous stop codons. Since no transcripts of Pat we re identified in different P. anserina strains grown under standard conditi ons and under increased stress, we conclude that none of the copies of Pat is active in the strains analyzed, under the environmental conditions inves tigated. Comparison of the sequences of the two cloned Pat sequences reveal ed 89% (589/747 nucleotides) identity. Most of the differences (82%, 129/15 8) can be attributed to transitions preferentially at CpA:TpG and CpT:ApG d inucleotides. The dinucleotide ratios in Pat are similar to those in a Neur ospora crassa transposon which was subject to repeat-induced mutation (RIP) , but differ significantly from those found in single-copy genes of P. anse rina and in fungal DNA transposons not modified by this mechanism. Molecula r analysis of the progeny of a cross between the wild-type strain and a tra nsgenic strain in which a nuclear gene was duplicated by transformation yie lded the first clear evidence that a RIP-like process is active in P. anser ina.