By sequence similarity, most sigma factors are σ-like (). They have four main regions (domains) that are generally conserved:
 N-terminus --------------------- C-terminus
The regions are further subdivided. For example, region 2 includes 1.2 and 2.1 through 2.4.
Domain 1.1 is found only in "primary sigma factors" (RpoD, RpoS in E.coli; "Group 1").  It is involved in ensuring the sigma factor will only bind the promoter when it is complexed with the RNA polymerase. Domains 2-4 each interact with specific promoter elements and with RNAP. Region 2.4 recognizes and binds to the promoter −10 element (called the "Pribnow box"). Region 4.2 recognizes and binds to the promoter −35 element.
Not every sigma factor of the σ family contains all the domains. Group 2, which includes RpoS, is very similar to Group 1 but lacks domain 1. Group 3 also lacks domain 1, and includes σ. Group 4, also known as the Extracytoplasmic Function (ECF) group, lack both σ1.1 and σ3. RpoE is a member.
Other known sigma factors are of the σ/RpoN () type. They are functional sigma factors, but they have significantly different primary amino acid sequences.