## 1 Introduction  
In bacteria, the initiation of transcription at promoters requires a dissociable specificity subunit of RNA polymerase (RNAP) called sigma (\(\sigma\)) that binds to the core (subunits \(\beta\beta\sigma\)\(\simeq\)0) to form the \"holoenzyme\". \(\sigma\) factors play distinct roles at different stages of initiation including the direct recognition of promoter elements to form an initial \"closed\" complex (RP\({}_{\rm c}\)), stabilisation of the \"open\" complex (RP\({}_{\rm o}\)) in which DNA around the transcription start site is melted, interaction with transcription activators, the stimulation of the early steps in RNA synthesis, and can influence promoter escape (reviewed in [1]). They can be classified into two distinct families based on their homology to two \(\sigma\) factors in _Escherichia coli_: the primary \(\sigma\) factor \(\sigma^{70}\) that is responsible for the bulk of transcription during growth; and the structurally unrelated \(\sigma^{54}\) (or \(\sigma^{\rm N}\)) that directs transcription in response to environmental signals, and requires the input of enhancer proteins and ATP hydrolysis to drive DNA melting (reviewed in [2]). This review focuses solely on the \(\sigma^{70}\) family. \(\sigma^{70}\) family members consist of up to four structurally conserved domains connected by flexible linkers that bind across one face of RNAP. In the holoenzyme, the DNA binding determinants of \(\sigma\) are exposed, allowing their interaction with promoter elements centered \(-35\) bp and \(-10\) bp upstream from the transcription start site. When RNAP escapes the promoter to enter the elongation phase, \(\sigma\) dissociates in stochastic fashion and only rebinds RNAP following termination, although there is evidence \(\sigma\) can associate with RNAP at transcription pause sites [3,4]. This \"\(\sigma\) cycle\" enables a regulatory strategy that involves the controlled production of alternative \(\sigma\) factors that can redirect RNAP to distinct promoters [5]. Following early discoveries of alternative \(\sigma\) factors involved in stress responses and sporulation in _Bacillus subtilis_[6,7] and the heat-shock response in _E. coli_[8], it became clear that the deployment of alternative \(\sigma\) factors to co-ordinately induce gene expression is widespread in bacteria. Indeed, large scale genome sequencing has revealed a plethora of \(\sigma\) factors in some organisms with, for example, 109 encoded by the Gram-negative myxobacterium _Sorangium cellulosum_[9]. The \(\sigma^{70}\) family has been classified into four major phylogenetically and structurally distinct groups with Group 1 consisting of primary \(\sigma\) factors and Groups 2-4 comprised of alternative \(\sigma\) factors with specialised functions. The expression and activity of alternative \(\sigma\) factors can be controlled at many levels, but particularly prevalent is their post-translational control by anti-\(\sigma\) factors that prevent their interaction with RNAP. Anti-\(\sigma\) factors use diverse mechanisms to release the \(\sigma\) in response to specific stimuli, often involving mechanisms that transduce extracellular signals to the cytoplasm. In this review, the structure and function of the \(\sigma^{70}\) family is considered, along with selected mechanisms for their control by anti-\(\sigma\) factors.