Transposition of MINE, a composite retrotransposon, in the avirulence gene ACE1 of the rice blast fungus Magnaporthe grisea

The ACE1 avirulence gene allele from the rice blast fungus Magnaporthe grisea was characterized in virulent isolate 2/0/3, revealing the insertion of a 1.9 kb MINE retrotransposon in the last ACE1 exon. MINE is a novel chimeric element composed of a transcribed non-coding sequence of 1.1 kb (WEIRD) fused to a 5′-truncated MGL retrotransposon. MINEs were found in high copy number in M. grisea isolates from rice (68 copies) and as a single copy in isolate CD156 from Eleusine. MINEs vary in size (1.3-6.7 kb) with conserved 5′ WEIRD sequences and variable 3′ MGL sequences. MGLs fused to WEIRDs correspond to different 5′-truncated MGLs with conserved 3′ ends. The organization and diversity of MINEs suggest that these retrotransposons result from independent fusions between WEIRD and 5′-truncated MGLs. Such chimera could be formed during MGL reverse transcription as proposed for human U6-LINE1 chimeric retrotransposons and integrated into M. grisea genome using MGL machinery.