Precision measurement of cis-regulatory energetics in living cells

Gene expression in all organisms is controlled by cooperative interactions between DNA-bound transcription factors (TFs). However, measuring TF-TF interactions that occur at individual cis-regulatory sequences remains difficult. Here we introduce a strategy for precisely measuring the Gibbs free energy of such interactions in living cells. Our strategy uses reporter assays performed on strategically designed cis-regulatory sequences, together with a biophysical modeling approach we call "expression manifolds". We applied this strategy in Escherichia coli to interactions between two paradigmatic TFs: CRP and RNA polymerase (RNAP). Doing so, we consistently obtain measurements precise to ~0.1 kcal/mol. Unexpectedly, CRP-RNAP interactions are seen to deviate in multiple ways from the prior literature. Moreover, the well-known RNAP binding motif is found to be a surprisingly unreliable predictor of RNAP-DNA binding energy. Our strategy is compatible with massively parallel reporter assays in both prokaryotes and eukaryotes, and should thus be highly scalable and broadly applicable