Revisiting the model for coactivator recruitment: Med15 can select its target sites independent of promoter-bound transcription factors

Activation domains (ADs) within transcription factors (TFs) induce gene expression by recruiting coactivators to specific regulatory regions. Within the prevailing model, TF-coactivator recruitment is independent of DNA binding, which is consistent with direct AD-coactivator interactions seen outside cells. However, this independence was not yet tested within the genomic context. Here, we targeted two Med15-interacting ADs to hundreds of budding yeast promoters through fusions with multiple DNA binding domains (DBDs), gradually controlling their abundances using libraries of synthetic promoters. Genomic profiling revealed that AD identity influences DNA binding locations and that transcription induction and Med15 recruitment are restricted to a subset of DBD-bound promoters displaying flexible expression, multiple-TFs binding, and fuzzy nucleosome architecture. Further, when fused to a DBD, Med15 redirected binding towards promoters of fuzzy nucleosomes, overcoming DBD-based preferences. Our results demonstrate that ADs and their recruited coactivators posses an inherent preference for genomic localization and, therefore, define the subset of induced promoters.转录因子（transcription factors, TFs）内的激活结构域（Activation domains, ADs）可通过将共激活因子招募至特定调控区域来诱导基因表达。在当前主流模型中，转录因子-共激活因子的招募过程与DNA结合相互独立，这与细胞外观察到的激活结构域-共激活因子直接互作结果一致。然而，这一独立性尚未在基因组环境中得到验证。本研究通过将两种结合Med15的激活结构域与多种DNA结合结构域（DNA binding domains, DBDs）融合，将其靶向至数百个酿酒酵母启动子区域，并借助合成启动子文库逐步调控其表达丰度。基因组谱分析结果显示，激活结构域的特性会影响DNA结合位点的选择；转录诱导与Med15招募仅局限于一类特定的DBD结合启动子亚群，这类启动子具有表达弹性、多转录因子结合以及模糊核小体结构特征。进一步研究发现，当与DNA结合结构域融合后，Med15会将结合重定向至模糊核小体富集的启动子区域，从而突破DNA结合结构域本身的结合偏好。本研究结果证实，激活结构域及其招募的共激活因子对基因组定位具有固有偏好性，并由此界定了被诱导的启动子亚群。