Sigma factor (σ factor) is an initiation factor that binds to RNA polymerase and promotes attachment to promoter sites on DNA.

Structure

Sigma factor has four main regions:

N----------C

1  2  3  4

• Region 1 regulates interaction with DNA.
• Region 2 binds RNAP and binds to the -10 promoter site.
• Region 4 binds to the -35 promoter site.