Figure 9. Pervasive biochemical activities on ‘off-target’ loci may represent molecular exaptations that generate novel edges between nodes of a standing gene regulatory networks (GRN).
Functional and nonfunctional interactions of Hairy with chromatin are depicted. (A) Hairy repression of target genes results in loss of active histone marks such as H4Ac, H3K27Ac, and H3K4me1 (dark gray peaks; gene x). Hairy interacts with many other nonfunctional targets where it carries out biochemical activities similar to those seen on transcriptionally controlled loci (gene y). The latter chromatin changes are inconsequential and unlikely to be evolutionarily selected. (B) Gain of activator sites in a region of Hairy-modified chromatin may generate an on/off switch and result in functional targeting. (C) Schematic representation of cooption of Hairy physical interaction into modified GRN.