Analysis of the promoter from an expanding mouse retrotransposon subfamily

The mouse genome contains several subfamilies of the retrotransposon L1. One subfamily, T-F, contains 4000-5000 full-length members and is expanding due to retrotransposition of a large number of active elements. Here we studied the T-F 5' untranslated region (UTR), which contains promoter activity required for subfamily expression. Using reporter assays, we show that promoter activity is derived from T-F-specific monomer sequences and is proportional to the number of monomers in the 5' UTR. These data suggest that nearly all full-length T-F elements in the mouse genome are currently competent for expression. We aligned the sequences of 53 monomers to generate a consensus T-F monomer and determined that most T-F elements are truncated near a potential binding site for a transcription initiation factor. We also determined that much of the sequence variation among T-F monomers results from transition mutations at CpG dinucleotides, suggesting that genomic T-F 5' UTRs are methylated at CpGs. (C) 1999 Academic Press.