text: Different sigma factors are utilized under different environmental conditions.  These specialized sigma factors bind the promoters of genes appropriate to the environmental conditions, increasing the transcription of those genes.
  Sigma factors in E. coli:
  *σ(RpoD) – σ – the "housekeeping" sigma factor or also called as primary sigma factor (Group 1), transcribes most genes in growing cells.  Every cell has a "housekeeping" sigma factor that keeps essential genes and pathways operating. In the case of E. coli and other gram-negative rod-shaped bacteria, the "housekeeping" sigma factor is σ.  Genes recognized by σ all contain similar promoter consensus sequences consisting of two parts.  Relative to the DNA base corresponding to the start of the RNA transcript, the consensus promoter sequences are characteristically centered at 10 and 35 nucleotides before the start of transcription (−10 and −35).
  *σ (FecI) – the ferric citrate sigma factor, regulates the fec gene for iron transport and metabolism
  *σ (RpoE) – extreme heat stress response and the extracellular proteins sigma factor
  *σ (RpoF/FliA) – the flagellar synthesis and chemotaxis sigma factor
  *σ (RpoH) – the heat shock sigma factor, it is turned on when the bacteria are exposed to heat. Due to the higher expression, the factor will bind with a high probability to the polymerase-core-enzyme. Doing so, other heatshock proteins are expressed, which enable the cell to survive higher temperatures. Some of the enzymes that are expressed upon activation of σ are chaperones, proteases and DNA-repair enzymes.
  *σ (RpoS) – the starvation/stationary phase sigma factor
  *σ (RpoN) – the nitrogen-limitation sigma factor
  There are also anti-sigma factors that inhibit the function of sigma factors and anti-anti-sigma factors that restore sigma factor function.