Intersecting transcription networks constrain gene regulatory evolution

Epistatic interactions, whereby a mutation's effect is contingent on another mutation, have been shown to constrain evolution within single proteins, and how such interactions arise in gene regulatory networks has remained unclear; here the appearance of pheromone-response regulator binding sites in the regulatory DNA of the a-specific genes of Saccharomyces cerevisiae are shown to have required specific changes in a second pathway during the evolution from its common ancestor with Candida albicans.