Rho-dependent termination: a bacterial evolutionary capacitor for stress resistance.

Since the Modern Synthesis, interest has grown in resolving the "black box" between genotype and phenotype. Contained within this black box are highly plastic RNA and proteins with global effects on chromosome integrity and gene expression that serve as evolutionary capacitors - elements that enable the accumulation and buffering of genetic variation in normal conditions and reveal hidden genetic variation when induced by environmental stress. Discussion of evolutionary capacitors has primarily focused on eukaryotic translation factors and chaperones, such as Hsp90 and PSI+ prion. However, due to the coupling of transcription and translation in prokaryotes, transcription factors can be equally impactful in the modulation of gene expression and phenotypes. In this review, we discuss the prokaryotic transcription terminator Rho and how mutagenesis and plasticity of Rho influence epistasis, evolvability, and adaptation to stress in bacteria. We discuss the effects of variation in Rho generated by nature, laboratory mutagenesis, and experimental evolution; and how this variation is constrained or encouraged by Rho's extensive network of protein interactors. Exploring Rho's role as an evolutionary capacitor, along with identifying additional elements that can serve this function, can significantly advance our understanding of how organisms adapt to thrive in diverse environments.