Dissecting endogeneous genetic circuits from first principles

For the vast majority of genes in sequenced genomes, there is limited understanding of how they are regulated. Without such knowledge, it is not possible to perform a quantitative theory-experiment dialogue on how such genes give rise to physiological and evolutionary adaptation. One category of high-throughput experiments used to understand the sequence-phenotype relationship of the transcriptome is massively parallel reporter assays (MPRAs). However, to improve the versatility and scalability of MPRA pipelines, we need a "theory of the experiment" to help us better understand the impact of various biological and experimental parameters on the interpretation of experimental data. These parameters include binding site copy number, where a large number of binding sites may titrate away transcription factors, as well as overlapping binding sites, which may affect analysis of the mutual dependence between mutations in the regulatory region and expression levels. Here, we develop a computational pipeline that makes it possible to systematically explore how each parameter controls measured MPRA data. Specifically, we use equilibrium statistical mechanics in conjunction with predictive base-pair resolution energy matrices to predict expression levels of genes with mutated regulatory sequences and subsequently use mutual information to interpret synthetic MPRA data including recovering the expected binding sites. Our simulations reveal important effects of the parameters on MPRA data and we demonstrate our ability to optimize MPRA experimental designs with the goal of generating thermodynamic models of the transcriptome with base-pair specificity. Further, this approach enables careful examination of the mapping between mutations in binding sites and their corresponding expression profiles, a tool useful not only for better designing MPRAs, but also for exploring regulatory evolution.