Alternative Sigma Factors of RNA Polymerase as Master Regulators in the Pathogenic Spirochaete Leptospira interrogans

This review summarizes the current knowledge on the role of alternative σ factors in the highly invasive spirochaete Leptospira interrogans, the causative agent of leptospirosis. This globally distributed zoonosis affects both animals and humans, resulting in substantial public health and economic consequences. Together with the primary σ⁷⁰, alternative σ factors provide transcriptional flexibility essential for bacterial adaptation to environmental changes and host-pathogen interactions. Comparative genomic analyses have revealed that the L. interrogans genome encodes 14 σ factors, including one housekeeping σ⁷⁰-like factor and three types of alternative σ factors: σ⁵⁴, σ²⁸, and 11 predicted extracytoplasmic function (ECF, σᴱ-type) factors. This review discusses the characteristics of these regulators, with particular emphasis on the poorly understood ECF σ factors and their potential roles in gene regulation, adaptive responses, and pathogenicity.