This family contains the bacterial Sigma factor-binding protein Crl (approximately 130 residues long). This is a transcriptional regulator of the csgA curlin subunit gene for curli fibres that are found on the surface of certain bacteria [1]. These proteins bind to the sigma-S subunit of RNA polymerase, activating expression of sigma-S-regulated genes. It was initially suggested that Crl affects transcription initiation in vitro by other sigmas, such as sigma-70 and sigma-32. However, it is now established that Crl binds specifically to the alternative sigma factor S/RpoS and favors its association with the core RNAP (RNA polymerase), thereby increasing its activity, which in turn regulates general stress response that protects many Gram-negative bacteria from several harmful environmental conditions. Furthermore, sigma S factor/RpoS plays important roles in biofilm formation and virulence of the food-borne pathogen Salmonella enterica serovar Typhimurium [2].

1: The Crl protein activates cryptic genes for curli formation and fibronectin binding in Escherichia coli HB101. Arnqvist A, Olsen A, Pfeifer J, Russell DG, Normark S; Mol Microbiol 1992;6:2443-2452. PMID:1357528

2: Recent advances in the characterization of Crl, the unconventional activator of the stress sigma factor sigmaS/RpoS. Cavaliere P, Norel F; Biomol Concepts. 2016;7:197-204. PMID:27180360