Isolation and characterization of long terminal repeat (LTR) retrotransposon <I>Le.</I>RTn1 of the basidiomycete <I>Lentinula edodes</I>

We identified a long terminal repeat (LTR)-type retrotransposon, designated Le.RTn1 in the basidiomycetous mushroom Lentinula edodes. Le.RTn1 (6,2l3 bp) carried 474-bp LTRs and contained two internal overlapping open reading frames (ORFs) putatively encoding GAG (group-specific antigens), and PRO (protease) plus POL (reverse transcriptase, RNase H, and integrase). Translation of the second ORF was suggested to occur by ribosomal frameshifting downstream of the gag domain. L. edodes chromosomes seemed to carry more than ten copies of Le.RTn1 (and its derivative sequences). Quite a number of solo LTRs were considered to be present on the chromosomes. The LTR contained four consecutive eukaryotic promoter consensus TATA (like)-boxes, and two relatively long direct repeats that are present upstream or downstream of the TATA (like)-boxes. We cloned the L. edodes cDNA (641 bp) encoding a cytochrome P450 heme-binding domain, to 5' end of which the part of the LTR sequence is fused. The 5' end (corresponding to the transcription start point) of the cloned cDNA was mapped to be the position just downstream of the TATA (like)-boxes. The promoter activity of the LTR was examined with hygromycin-resistance gene as a reporter in the basidiomycete Coriolus hirsutus, showing a remarkable activity in the heterologous host. The LTR also contained the terminator consensus sequences TAG-----TAGT---TTT, first identified in yeast. To verify that these sequences do work as a transcription terminator, analysis of the 3' end of Le.RTn1 transcript was in progress.