Page "Sigma factor" Paragraph 1

Every molecule of RNA polymerase holoenzyme contains exactly one sigma factor subunit, which in the model bacterium Escherichia coli is one of those listed below.

The number of sigma factors varies between bacterial species.

E. coli has seven sigma factors.

Sigma factors are distinguished by their characteristic molecular weights.

For example, σ < sup > 70 </ sup > refers to the sigma factor with a molecular weight of 70 kDa.