Phase transition of bacterial single-stranded DNA binding (SSB) protein upon stress response and metabolic adaptation

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Abstract Single-stranded DNA-binding proteins (SSBs) are ubiquitous factors of genome metabolism, recently recognized for their ability to undergo liquid–liquid phase separation (LLPS). While Escherichia coli SSB (EcSSB) has emerged as a model for bacterial LLPS in vitro , its phase behavior and functional dynamics in vivo have remained largely unexplored. Here, we define the subcellular organization of EcSSB under diverse physiological and stress conditions using super-resolution microscopy coupled with newly developed image analysis tools. We show that EcSSB forms dynamic intracellular assemblies during exponential growth, which partially dissolve in response to DNA damage, oxidative stress, antibiotic exposure, and metabolic adaptation. In contrast, these dynamic reorganizations are attenuated in stationary phase cells. Moreover, we show that EcSSB foci exhibit limited overlap with nucleoid regions under stress-free conditions, whereas stress induction is accompanied by increased DNA association. These in vivo observations are consistent with stress- and growth-phase dependent modulation of EcSSB organization and together with biophysical characterization of EcSSB condensation in vitro suggest a role for LLPS-based dynamic storage and mobilization in response to physiological demands. Our work provides a quantitative framework for analyzing the cellular organization of bacterial proteins and the spatial regulation of genome maintenance factors in changing environments. This knowledge may also support future strategies targeting SSB organization pathways for antimicrobial development.

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