E. coli has seven different σ factors, which were initially distinguished based on their molecular masses and as described for σ⁷⁰, the names given included the molecular mass as a superscript.Two other nomenclature systems are also used. In the first, each σ factor is indicated by the Greek letter σ followed by a superscript letter. For example, σ⁷⁰ is called σ^D in this system. In the second system, each σ factor is named after the gene that codes for it. For example, σ⁷⁰ becomes RpoD. Each kind of σ factor combines with core polymerase to form a holoenzyme that recognizes a unique set of promoter DNA sequences. The properties and functions of E. coli sigma factors are summarized. All σ proteins bind to RNA polymerase core enzymes and to DNA, usually but not always recognizing two separate DNA sites. Many possibly all σ factors, also are targets of accessory ligands that regulate their activity.

    Based on sequence homologies, E. coli factors can be divided in two major families, the σ⁷⁰ family and the σ⁵⁴ (σ^N family). The σ⁵⁴ factor is the only member of the σ⁵⁴ family. All other E. coli sigma factors belong to the σ⁷⁰ family. E. coli σ factors compete with one another for the core RNA polymerase. In the absence of additional regulatory factors, the rate of transcription depends solely on promoter strength and the concentration of specific holoenzymes. The RNA polymerase holoenzyme that contains σ⁷⁰ transcribes genes that code for proteins required for normal exponential growth, which account for most of the bacteria's genes. Therefore, σ⁷⁰ is an essential protein. RNA polymerase holoenzymes containing one of the other σ factors recognize specific sets of genes that code for proteins needed for cell survival under some special environmental condition such as heat shock (σ^H) or nitrogen deprivation (σ^N).

    Sequences are known for more than one hundred members of the σ⁷⁰ family. Aligning sequences reveals four conserved regions, each of which can be subdivided into smaller highly conserved regions. Each conserved region except region 1. 1 is present in all sigma factors of the σ⁷⁰ family. Region 1. 1 is present only in the major sigma factor of each bacterial species, which for E. coli is σ⁷⁰ itself. RNA polymerase holoenzyme can bind to a few promoters that lack a-35 box. It can do so because these promoters have a supplementary sequence element 5'_TG-3'located one base up-stream from the-10 box known as the extended-10 element. The σ⁷⁰ factor makes specific contacts with this element.

    Sigma factors in other kinds of bacteria sometimes have novel functions. For instance, four sporulation-specific sigma factors allow Bacillus subtilis to respond to certain types of nutrient deprivation by becoming metabolically dormant cells or spores that are surrounded by a protective multilayer envelope. Spore formation is a complex process, requiring many enzymes and proteins that are not synthesized by exponentially growing cells. During spore formation, preexisting sigma subunits are destroyed and new sigma subunits formed, which combine with the core enzyme to form a holoenzyme that recognizes promoters of genes needed for spore formation.