Gene regulation by extracytoplasmic function (ECF) σ factors in alpha-rhizobia

Bacteria have evolved efficient mechanisms to sense environmental changes to which they respond with high sensitivity and specificity. Reprogramming gene transcription by directing RNA polymerase to specific promoters through alternative σ subunits is a wide-spread bacterial strategy of rapid adaptation to environmental challenges. Extracytoplasmic function (ECF) σ factors represent the most minimalistic and diverse group of the σ⁷⁰ protein family. These σ factors are typically stress-inducible and non-essential. Most bacterial species possess multiple ECF σ factors. Here, we review the current knowledge on promoter specificity, regulatory control and biological function of ECF σ factors in members of the α-proteobacterial Rhizobiales, which enter a root nodule symbiosis with leguminous host plants.