Structural remodeling and activation of the hdeAB and proVWX operons in response to pH and salt stress.

Escherichia coli is highly sensitive to acid and osmotic stress but adapts by modulating the expression of stress-responsive genes. Nucleoid-associated proteins (NAPs) play key roles in DNA organization and sensing environmental changes. The histone-like nucleoid structuring protein H-NS is an NAP acting as a global regulator of stress genes. H-NS may alter local chromatin structure to modulate the expression of such genes in response to environmental stress. The H-NS homolog suppressor of td phenotype A (StpA) co-regulates several target genes, but its role is poorly defined. To investigate the regulatory interplay between these two proteins, we examined transcription, DNA binding, and chromatin structure at two regulated operons, hdeAB and proVWX, in E. coli following exposure to acid and salt shock. Our results show that H-NS senses pH and osmotic cues to remodel chromatin and relieve repression, while StpA-in part-compensates for H-NS loss, particularly at proVWX, highlighting a coordinated regulatory network.