Figure 2.

Expression flexibility, mediated by promoter architecture, may be due to increased dependence on trans regulation and environmental changes. Genes with a TATA box, promoter occupied with nucleosomes and many binding sites are regulated more extensively by regulatory factors. These factors respond to extracellular signals, thus making the target genes responsive to environmental changes both on short timescales (responsiveness and noise) as well as on longer timescales (evolutionary changes). These flexible genes preferentially code for proteins that interact with the environment and mediate the response to environmental changes (curved arrow), and this may allow for rapid adaptation to new environments.