abstract: In order to study biomolecular mechanisms and rationally design small molecule inhibitorsactivators, it is important to study biological molecules through their structure at atomic resolution. Classical structural biology techniques have allowed the reconstruction of the 3D structure of many important biological molecules at atomic resolution level. However, in order to function properly, biological molecules are able to intrinsically and dynamically interconvert between many different structural forms and topologies at a multitude of rates. One out of a myriad of examples is the case of the bacterial RNAP polymerase complex with a DNA promoter sequence and its ability to initiate DNA transcription not from a specific site on DNA but rather from many different possible ones. There is currently only one crystal structure of the bacterial transcription initiation complex at this stage of DNA transcription, however, single-molecule Forster resonance energy transfer (smFRET) measurements have shown the complex at this stage dynamically interconverts between two conformational states. In this talk I have shown how the transcription initiation complex dynamically interconvert (in microseconds) between two conformational states, where one is characterized by the existing crystal structure, whereas the other represents an unsolved structure. I will show how by combining structural simulations constrained by spatial constraints derived from multiple single-molecule fluorescence-based measurements, I solve the ensemble structures that have the highest likelihood to represent the second unresolved conformational state of the bacterial transcription initiation complex at atomic resolution. This talk showcases how molecular biophysics and modeling can be combined to resolve realistic representations of dynamic conformations.