The σ(70) family of sigma factors

Members of the σ(70 )family of sigma factors are components of the RNA polymerase holoenzyme that direct bacterial or plastid core RNA polymerase to specific promoter elements that are situated 10 and 35 base-pairs upstream of transcription-initiation points. Members of the σ(70 )family also function as contact points for some activator proteins, such as PhoB and λcl, and play a role in the initiation process itself. The primary σ factor, which is essential for general transcription in exponentially growing cells, is reversibly associated with RNA polymerase and can be replaced by alternative σ factors that co-ordinately express genes involved in diverse functions, such as stress responses, morphological development and iron uptake. On the basis of gene structure and function, members of the σ(70 )family can broadly be divided into four main groups. Sequence alignments of the σ(70 )family members reveal that they have four conserved regions, although the highest conservation is found in regions 2 and 4, which are involved in binding to RNA polymerase, recognizing promoters and separating DNA strands (so-called 'DNA melting'). The division of the linear sequence of σ(70 )factors into four regions is largely supported by recent structural data indicating that primary σ factors have three stable domains that incorporate regions 2, 3 and 4. Furthermore, structures of the RNA polymerase holoenzyme have revealed that these domains of σ(70 )are spread out across one face of RNA polymerase. These structural data are starting to illuminate the mechanistic role of σ factors in transcription initiation.