In the human genome, retrotranspositionally competent long interspersed nuclear elements (L1Hs) are involved in the generation of processed pseudogenes and mobilization of unrelated sequences into existing genes. Transcription of each L1Hs is initiated from its internal promoter but may also be driven from the promoters of adjacent cellular genes. Here I show that a hitherto unknown L1Hs antisense promoter (ASP) drives the transcription of adjacent genes. The ASP is located in the L1Hs 5 untranslated region (5 UTR) and works in the opposite direction. Fifteen cDNAs, isolated from a human NTera2D1 cDNA library by a differential screening method, contained L1Hs 5 UTRs spliced to the sequences of known genes or non-proteincoding sequences. Four of these chimeric transcripts, selected for detailed analysis, were detected in total RNA of different cell lines. Their abundance accounted for roughly 1 to 500% of the transcripts of four known genes, suggesting a large variation in the efficiency of L1Hs ASP-driven transcription. ASP-directed transcription was also revealed from expressed sequence tag sequences and confirmed by using an RNA dot blot analysis. Nine of the 15 randomly selected genomic L1Hs 5 UTRs had ASP activities about 7-to 50-fold higher than background in transient transfection assays. ASP was assigned to the L1Hs 5 UTR between nucleotides 400 to 600 by deletion and mutation analysis. These results indicate that many L1Hs contain active ASPs which are capable of interfering with normal gene expression, and this type of transcriptional control may be widespread.

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Transcription of transposable elements interspersed in the genome is controlled by complex interactions between their regulatory elements and host factors. However, the same regulatory elements may be occasionally used for the transcription of host genes. One such example is the human L1 retrotransposon, which contains an antisense promoter (ASP) driving transcription into adjacent genes yielding chimeric transcripts. We have characterized 49 chimeric mRNAs corresponding to sense and antisense strands of human genes. Here we show that L1 ASP is capable of functioning as an alternative promoter, giving rise to a chimeric transcript whose coding region is identical to the ORF of mRNA of the following genes: KIAA1797, CLCN5, and SLCO1A2. Furthermore, in these cases the activity of L1 ASP is tissue-specific and may expand the expression pattern of the respective gene. The activity of L1 ASP is tissue-specific also in cases where L1 ASP produces antisense RNAs complementary to COL11A1 and BOLL mRNAs. Simultaneous assessment of the activity of L1 ASPs in multiple loci revealed the presence of L1 ASP-derived transcripts in all human tissues examined. We also demonstrate that L1 ASP can act as a promoter in vivo and predict that it has a heterogeneous transcription initiation site. Our data suggest that L1 ASP-driven transcription may increase the transcriptional flexibility of several human genes.

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BackgroundTranscriptional interference has been recently recognized as an unexpectedly complex and mostly negative regulation of genes. Despite a relatively few studies that emerged in recent years, it has been demonstrated that a readthrough transcription derived from one gene can influence the transcription of another overlapping or nested gene. However, the molecular effects resulting from this interaction are largely unknown.Methodology/Principal FindingsUsing in silico chromosome walking, we searched for prematurely terminated transcripts bearing signatures of intron retention or exonization of intronic sequence at their 3′ ends upstream to human L1 retrotransposons, protein-coding and noncoding nested genes. We demonstrate that transcriptional interference induced by intronic L1s (or other repeated DNAs) and nested genes could be characterized by intron retention, forced exonization and cryptic polyadenylation. These molecular effects were revealed from the analysis of endogenous transcripts derived from different cell lines and tissues and confirmed by the expression of three minigenes in cell culture. While intron retention and exonization were comparably observed in introns upstream to L1s, forced exonization was preferentially detected in nested genes. Transcriptional interference induced by L1 or nested genes was dependent on the presence or absence of cryptic splice sites, affected the inclusion or exclusion of the upstream exon and the use of cryptic polyadenylation signals.Conclusions/SignificanceOur results suggest that transcriptional interference induced by intronic L1s and nested genes could influence the transcription of the large number of genes in normal as well as in tumor tissues. Therefore, this type of interference could have a major impact on the regulation of the host gene expression.