The sigma factor cascade in the formation of flagella is important in assuring the correct and successful assembly of the protein complex. σ28 is used to induce the synthesis of the third stage in the cascade, ensuring the correct proteins are present at the right time. σ28 is activated by fliA, a protein produced in the second step of the cascade.
Temporal control of gene expression is maintained, as each stage must be complete before the next one is activated.