Protein: Protein Interactions Control Sensitivity Of A Transcription Response To Input Signal

Transcription regulation frequently occurs through multi-step assembly processes that can include small ligand binding, protein-protein and protein-nucleic acid interactions. However, for the majority of these assembly processes the relationship between these coupled equilibria and the sensitivity of the transcription response to the regulatory input signal is not known. The E. coli biotin operon repression complex assembly, which responds to input biotin concentration, occurs via three coupled equilibria including corepressor binding, holorepressor dimerization and dimer binding to DNA. A genetic screen has yielded superrepressor mutants that repress biotin operon transcription in vivo at biotin concentrations much lower than that required by the wild type repressor. In this work, isothermal titration calorimetry and sedimentation equilibrium measurements were used to determine the superrepressor biotin binding and homodimerization properties. The results indicate that, although all variants exhibit biotin binding affinities similar to that measured for BirAwt, five of the six superrepressors show altered homodimerization energetics. Molecular dynamics simulations predict complex structural origins of the altered dimerization. Modeling of the multi-step repression complex assembly process for these proteins reveals that the altered sensitivity of the transcription response to biotin concentration is readily explained solely by the altered superrepressor homodimerization energetics. These results highlight how a transcription regulatory response to input signal can be fine-tuned via linked equilibria.