Abstract
Single-molecule detection and manipulation is a powerful tool for unraveling dynamic biological processes. Unfortunately, success in such experiments is often challenged by tethering the biomolecule(s) of interest to a biocompatible surface. Here, we describe a robust surface passivation method by dense polymer brush grafting, based on optimized polyethylene glycol (PEG) deposition conditions, exactly at the lower critical point of an aqueous biphasic PEG-salt system. The increased biocompatibility achieved, compared with PEG deposition in sub-optimal conditions away from the critical point, allowed us to successfully detect the assembly and function of a large macromolecular machine, a fluorescent-labeled multi-subunit, human RNA Polymerase II Transcription Pre-Initiation Complex, on single, promoter-containing, surface-immobilized DNA molecules. This platform will enable probing the complex biochemistry and dynamics of large, multi-subunit macromolecular assemblies, such as during the initiation of human RNA Pol II transcription, at the single-molecule level.