Sigma Factors in Gene Expression

Abstract Sigma (σ) factors control the promoter selectivity of bacterial RNA polymerase (RNAP). On binding to RNAP, σ factors allow efficient promoter recognition and transcription initiation. Bacterial promoters are typically comprised of two hexameric deoxyribonucleic acid (DNA) sequences located approximately 10 and 35 bases upstream of the transcription start site (the −10 and −35 element, respectively). All bacteria contain a primary σ factor that is responsible for transcription of housekeeping genes necessary for growth and survival. In addition, many bacteria encode multiple alternative σ factors. The level and activity of the alternative σ factors are highly regulated and can vary depending on environmental or developmental signals. The synthesis of alternative σ factors allows the coordinated activation of discrete sets of genes through the recognition of distinct promoter sequences and thereby contributes to stress responses, motility, endospore formation and numerous other adaptive responses. Key Concepts: σ Factors are necessary for promoter recognition by RNA polymerase and efficient transcription initiation. Transcription initiation is a highly regulated, multistep process. σ Factors can be grouped into two families on the basis of their similarity to the housekeeping σ factor, σ 70 , or the nitrogen‐responsive σ factor, σ 54 . Members of the σ 70 family generally contain four conserved domains (regions 1–4). Most bacteria encode multiple alternative σ factors that direct transcription of genes in response to environmental cues and developmental transitions. Alternative σ 70 family proteins often lack region 1 and sometimes region 3 (minimally containing conserved regions 2 and 4). σ Factor activity can be regulated at many levels, such as through sequestration by anti‐σ factors or by protolytic activation.