Bidirectional Gene Pairs in the Human Genome

Bidirectional gene pairs, also called head-to-head gene pairs, are defined as two genes on different strands with adjacent 5′-ends. They are transcribed divergently in a variety of coordinated fashion. The region between a bidirectional gene pair is designated as a putative bidirectional promoter, which is suggested to coordinately regulate the expression of the pair. The closely located bidirectional gene pairs and their bidirectional promoters were recently found to be enriched in the human genome. Despite substantial interests in these gene pairs and their expression regulation through bidirectional promoters, their biological significance is not well understood. Furthermore, it is of particular interest to know when and how the bidirectional gene arrangement evolved. Here the current understanding of these pairs is reviewed: bidirectional gene pairs may have great biological significance, and the bidirectional gene arrangement has evolved during the evolution of vertebrates leading to mammals by relocation of existing genes.