Background Long interspersed element type 1 (LINE-1) are highly abundant non-LTR retrotransposons in mammalian genomes. LINE-1 hypomethylation and overexpression has been considered as a hallmark of many human cancers. Moreover, active movement of LINE-1 results in insertional mutagenesis and may contribute to cancer initiation or progression. Transcription is the initial and critical regulatory step for active LINE-1 movement. Therefore, investigating LINE-1 transcriptional regulation is important for our understanding of LINE-1’s role in cancer. In human LINE-1, an intact YY1 transcription factor binding motif is required for accurate transcription initiation. Two subfamilies of mouse LINE-1, Tf_I and Gf_I, have predicted YY1 binding motifs in their promoter region but their role in mouse LINE-1 transcription has not been characterized. In this study, we aim to investigate the role of these putative YY1 motifs in Tf_I and Gf_I subfamilies.
Methods and Results Promoter sequences carrying the wild-type or mutant YY1 motifs were cloned into a dual-luciferase reporter system. Reporter assays showed transcription inactivation upon mutation of the YY1 motif for the Tf_I subfamily. The electrophoretic mobility shift assay (EMSA) demonstrated specific interaction of the Tf_I promoter sequence with the YY1 protein. Knockdown of YY1 protein level in cells reduced Tf_I promoter activity by 2-fold compared to the wild-type control. These results indicate that YY1 is a transcriptional activator of mouse LINE-1 Tf_I subfamily. On the other hand, our data suggest that the predicted YY1 binding site in the promoter of Gf_I subfamily is not functional due to one nucleotide deviation from the consensus YY1 motif. Indeed, restoring it to the conserved YY1 motif sequence enhanced the promoter activity by 6-fold. In EMSA, YY1 protein did not interact with the putative YY1 motif of the Gf_I subfamily but did so when the motif was restored to consensus YY1 sequence.
Conclusions Findings from our study increase our understanding of the transcriptional regulation of different mouse LINE-1 subfamilies, highlight the differing roles of YY1 in regulating human and mouse L1 transcription, and have important implications in the role of LINE-1s during cancer development.
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