Question

How is switching sigma factors used to control gene expression? What type of organisms use this method

Answer 1

Sigma factors provide promoter recognition specificity to RNA polymerase holoenzyme, contribute to DNA strand separation, and then dissociate from the core enzyme following transcription initiation. As the regulon of a single sigma factor can be composed of hundreds of genes, sigma factors can provide effective mechanisms for simultaneously regulating expression of large numbers of prokaryotic genes. One newly emerging field is identification of the specific roles of alternative sigma factors in regulating expression of virulence genes and virulence-associated genes in bacterial pathogens. Virulence genes encode proteins whose functions are essential for the bacterium to effectively establish an infection in a host organism. In contrast, virulence-associated genes can contribute to bacterial survival in the environment and therefore may enhance the capacity of the bacterium to spread to new individuals or to survive passage through a host organism. As alternative sigma factors have been shown to regulate expression of both virulence and virulence-associated genes, these proteins can contribute both directly and indirectly to bacterial virulence. Sigma factors are classified into two structurally unrelated families, the ?70 and the ?54 families. The ?70family includes primary sigma factors (e.g., Bacillus subtilis ?A) as well as related alternative sigma factors; ?54 forms a distinct subfamily of sigma factors referred to as ?N in almost all species for which these proteins have been characterized to date.

Sigma factors are a class of proteins constituting essential dissociable subunits of prokaryotic RNA polymerase. The association of appropriate alternative sigma factors with core RNA polymerase provides a mechanism for cellular responses mediated through redirection of transcription initiation. Sigma factors provide promoter recognition specificity to the polymerase and contribute to DNA strand separation; they then dissociate from RNA polymerase core enzyme following transcription initiation . As the regulon of a single sigma factor can be comprised of hundreds of genes, sigma factors provide effective mechanisms for simultaneously regulating large numbers of prokaryotic genes. In some cases, the genes comprising a sigma factor regulon have a clearly defined primary function (e.g., genes regulated by the sporulation sigma factors in Bacillus subtilis ; in others, the genes comprising a regulon contribute to multiple functions (e.g., the stationary-phase and general stress response genes regulated by ?B in Listeria monocytogenes

The general stress-responsive alternative sigma factors ?S (RpoS) and ?B transcribe genes contributing to bacterial survival under conditions of environmental stress in gram-negative and in gram-positive bacteria, respectively . ?S was identified in both Escherichia coli and S. enterica serovar Typhimurium as an alternative sigma factor that activates the expression of numerous genes required to maintain cell viability during stationary phase . ?S also plays a key role in protecting E. coli and S. entericaserovar Typhimurium from different environmental stress conditions, including starvation, hyperosmolarity, oxidative damage, and reduced pH . Since its initial discovery, the presence of ?S and its role in the stress response has been confirmed in many gram-negative bacterial species, including P. aeruginosa, Borrelia burgdorferi, and Vibrio cholerae . Through enhancing environmental survival, as well as by directly activating virulence genes, ?B and ?S have both direct and indirect roles in bacterial pathogenesis.  

Bacteria or rather prokaryotes use this method of regulating transcription.