Fitness consequences of a regulatory polymorphism in a seasonal environment.

Gene regulation is commonly assumed to have evolved in response to environmental variability. Although tightly regulated in Escherichia coli strain K12, transcriptional control of arginine biosynthesis is deregulated in strain B. Caused by a single amino acid replacement in the arginine repressor, these contrasting regulatory strategies result in a fitness tradeoff. The K12 repressor is selectively favored in the presence of arginine and disfavored in its absence. In environments that cycle between high and low arginine, short seasons favor the K12 allele, whereas long seasons favor the B allele. Unexpectedly then, deregulated expression is adaptive in some seasonal habitats.