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