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. 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 ( 16). 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 are a class of proteins constituting essential dissociable subunits of prokaryotic RNA polymerase. For each sigma factor, when applicable, examples are drawn from multiple species. We present several examples of alternative sigma factors that have been shown to contribute to virulence in at least one organism. The σ 70 family includes primary sigma factors (e.g., Bacillus subtilis σ A) as well as related alternative sigma factors σ 54 forms a distinct subfamily of sigma factors referred to as σ N in almost all species for which these proteins have been characterized to date. Sigma factors are classified into two structurally unrelated families, the σ 70 and the σ 54 families. 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. 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. Virulence genes encode proteins whose functions are essential for the bacterium to effectively establish an infection in a host organism. 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. 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. 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.
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