For a significant fraction of mRNAs, their expression is regulated by other RNAs, including cis natural antisense transcripts (cis -NATs) that are complementary mRNAs transcribed from opposite strands of DNA at the same genomic locus. The regulatory mechanism of mRNA expression by cis -NATs is unknown, although a few possible explanations have been proposed. To understand this regulatory mechanism, we conducted a large-scale analysis of the currently available data and examined how the overlapping arrangements of cis -NATs affect their expression level. Here, we show that for both human and mouse the expression level of cis -NATs decreases as the length of the overlapping region increases. In particular, the proportions of the highly expressed cis -NATs in all cis -NATs examined were ∼36 and 47% for human andmouse, respectively, when the overlapping region was <200 bp. However, both proportions decreased to virtually zero when the overlapping regions were >2000 bp in length. Moreover, the distribution of the expression level of cis -NATs changes according to different types of the overlapping pattern of cis -NATs in the genome. These results are consistent with the transcriptional collision model for the regulatory mechanism of gene expression by cis -NATs.
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