Forewarned is forearmed: exploring extracytoplasmic function sigma factors as molecular sentinels in bacterial stress response and signal transduction

Abstract Bacterial adaptation in complex environments is contingent upon the rapid perception of stress through sensory and regulatory domains, which subsequently activate a diverse array of signaling factors. Many Phytobacteria rely on Extracytoplasmic Function (ECF) sigma factors, the largest and most diverse group of alternative sigma factors. ECF sigma factors direct RNA polymerase to specific promoters, activating discrete regulons in response to external cues. In phytobacteria, they are master regulators of virulence, controlling key processes such as the deployment of type III secretion systems, the biosynthesis of exopolysaccharides, and resistance to host-derived oxidative stress. Most bacterial species harbor a repertoire of sigma factors, with the largest group being the ECF family. This family is characterized by its distinction from σ 70 and is identified by shared domain structures that interact with promoters to regulate signal transduction. They are widespread in phytobacteria, especially σ E from Escherichia coli , AlgU from Pseudomonas aeruginosa , HrpL from Pseudomonas syringae , SigX from Bacillus subtilis , and FecI from E. coli . This review synthesizes the diverse regulatory mechanisms that control ECF sigma factor activity in plant-pathogenic bacteria, focusing on canonical anti-sigma factor sequestration, partner-switching cascades, and phosphorylation-dependent pathways. Additionally, we elucidate the regulatory mechanisms associated with ECF sigma factors and their integration with one-component and two-component systems (OCS and TCS). Furthermore, we examine both soluble and insoluble alternative anti-sigma factors at the transcriptional level in response to stimulus perception. A deeper understanding of ECF-dependent signal transduction is anticipated to broaden our comprehension of the diversity inherent in bacterial signal transduction, thereby laying a foundation for future experimental research.