Distance of promoters and enhancers to evolutionary gene classes. (A) Distance cumulative frequency distribution of the nearest promoter, or (B) enhancer (active and poised) to transcription start sites (TSSs) from each evolutionary gene category. (C) Model of new gene transcriptional regulation. Enhancers exhibit bidirectional transcription, which can lead to de novo gene expression, or expression of duplications/insertions. If the new gene provides a useful function, selection will occur on not only protein function, but also the gene structure leading to more exons, and on regulatory elements to provide more temporal or spatial control, and more or less transcription. Ultimately, evolution on enhancer sequences will convert it to a traditional promoter.
