This group dissipated from the ECF classification during the reclassification by [[Casas-Pastor et al.]]!

Taxonomical distribution

The large majority of ECF20 proteins belong to organisms of the phylum Proteobacteria. But a few ECF20 σ factors were also identified in organisms of the phylum Acidobacteria (Staroń et al., 2009).

Experimentally studied examples

Two members of this group, CnrH from Cupriavidus metallidurans (Grosse et al., 2007) and MibX from Microbispora coralina (Foulston and Bibb, 2011), have been experimentally addressed so far.

Genomic context conservation

The genes coding for ECF20 σ factors are associated with genes encoding small membrane-anchored CAS domain containing anti-σ factors or alternatively, in some cases, longer genes encoding proteins containing ‘von Willebrand factor-A’ domains (Staroń et al., 2009).

Signaling mechanism

In the presence of the inducing stimulus, which in the case of the CnrH anti-σ factor CnrX is the binding of cobalt or zinc to its extracytoplasmic domain, the anti-σ factor undergoes a conformational change that ultimately leads to the release of the σ factor (Grosse et al., 2007). This is in turn free to initiate transcription from its target promoters. Remarkably, the CnrH/CnrX interaction structurally resembles that of the NepR/EcfG pair of group ECF15, indicating similar inhibitory mechanism (Maillard et al., 2014).

Physiological function

In C. metallidurans, CnrH was implicated in nickel resistance (Grosse et al., 2007), while MibX in M. coralina has a role in antibiotic production (Foulston and Bibb, 2011).


Members of original ECF20 are scattered across several groups from the new classification. The largest contribution of original ECF20 is to new groups ECF289 (42.27%), ECF290 (95.41%) and ECF291 (33.76%).


Foulston L, Bibb M. 2011. Feed-forward regulation of microbisporicin biosynthesis in Microbispora corallina. Journal of Bacteriology 193 (12): 3064–3071.
Grosse C, Friedrich S, Nies DH. 2007. Contribution of extracytoplasmic function sigma factors to transition metal homeostasis in Cupriavidus metallidurans strain CH34. Journal of Molecular Microbiology and Biotechnology 12 (3-4): 227–240.
Maillard AP, Girard E, Ziani W, Petit-Härtlein I, Kahn R,et al. 2014. The crystal structure of the anti-σ factor CnrY in complex with the σ factor CnrH shows a new structural class of anti-σ factors targeting ExtraCytoplasmic-Function σ factors. Journal of Molecular Biology 426 (12): 2313–2327.
Rhodius VA, Segall-Shapiro TH, Sharon BD, et al. 2013. Design of orthogonal genetic switches based on a crosstalk map of σs, anti-σs, and promoters. Molecular Systems Biology 9:702.
Staroń A, Sofia HJ, Dietrich S, Ulrich LE, Liesegang H, et al. 2009. The third pillar of bacterial signal transduction: classification of the extracytoplasmic function (ECF) sigma factor protein family. Molecular Microbiology 74 (3): 557–581.
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