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

Taxonomical distribution

σ factors from this group were identified in organisms of the phyla Acidobacteria, Nitrospira, Proteobacteria and Verrucomicrobia (Gómez-Santos et al., 2011).

Experimentally studied examples

The ECF44 σ factor CorE of M. xanthus was studied regarding both its physiological role and the mechanism of signal transduction (Gómez-Santos et al., 2011).

Genomic context conservation

Genes encoding the ECF44 σ factors are not linked to any obvious anti-σ factors. They lack strong genomic context conservation but are usually associated with genes encoding proteins putatively involved in copper handling and trafficking (Gómez-Santos et al., 2011).

Signaling mechanism

These σ factors contain a C-terminal extension that, similarly to what was demonstrated for ECF41 σ factors, seems to play a dual role; it contributes both to their activation and inactivation. The C-terminal extension of CorE senses the redox state of copper by a mechanism not yet completely understood and regulates σ factor activity (Gómez-Santos et al., 2011). The Cys arrangement of the Cys-rich domain at the C terminus determines the metal specificity. Moreover, the conserved CxC motif located between the sigma 2 domain and the sigma 4.2 region has also been found to be essential for activity (Marcos-Torres et al., 2016)

Physiological function

These σ factors are involved in regulating copper ion homeostasis (Gómez-Santos et al., 2011).


Members of original group ECF44 are present in ECF238, together with members of original ECF24. 13.09% of the members of ECF238 have homology to ECF44.


Gómez-Santos N, Pérez J, Sánchez-Sutil MC, Moraleda-Muñoz A, Muñoz-Dorado J. 2011. CorE from Myxococcus xanthus is a copper-dependent RNA polymerase sigma factor. PLoS Genetics 7 (6): e1002106.
Marcos-Torres FJ, Pérez J, Gómez-Santos N, Moraleda-Muñoz A, Muñoz-Dorado J. 2016. In depth analysis of the mechanism of action of metal-dependent sigma factors: characterization of CorE2 from Myxococcus xanthus. Nucleic Acids Res. 44:5571–84.
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