Sigma factors are used control large groups of genes – regulons, by changing the binding specificity of RNA polymerase. This allows specific regulons to be upregulated in certain growth conditions, improving the survival chances of the cell.
For example, σ70 is the standard log-growth phase σ factor. This upregulates the expression of the largest regulon in E. coli, allowing the fastest growth of the bacteria during favourable conditions (high nutrient concentration, optimal temperature).
σ32 is the sigma factor involved in the E. coli heat-shock response. This upregulates the expression of protein chaperones, stabilising proteins at higher temperatures to still allow cellular function. Specific proteases also have their expression upregulated, degrading any denatured proteins to prevent the cytoplasm from filling up with dysfunctional proteins.
σ28 is a sigma factor found in the sigma cascade of flagellar formation. After the first stage of assembly has finished, σ28 is activated, allowing the next step to begin. By using a cascade, the formation of the flagellar will be conducted in the correct process, leading to a functional protein complex. Without this cascade, it would not be possible to assemble the flagellar motor or fibre, due to the complexity of the protein.
σ54 controls nitrogen-response genes. This sigma factor does not have any anti-sigma factors; this is likely due to the convergent evolution that occurred during its formation. The induction of σ54 increases uptake of nitrogen from the environment.
σ38 controls the regulon necessary for the stationary phase of growth. This is much smaller than the regulon controlled by σ70, as fewer genes are necessary when there is less growth.