Activation and repression of transcription initiation in bacteria.

Transcription initiation is the principal step at which bacterial gene expression is regulated. Bacterial transcription is due to a single multisubunit RNA polymerase. The potential transcription initiation rate of any promoter is set by the efficiency with which RNA polymerase recognizes the different promoter sequence elements. The sigma subunit plays the major role in the process of promoter recognition. Different RNA polymerase sigma subunits can guide RNA polymerase to different promoters. The E. coli genome encodes seven different sigma subunits, each of which allows the cell to respond to different environmental stimuli. A large number of transcription factors up-regulate and down-regulate expression from different promoters in response to environmental signals. Many transcription activators function by making a direct interaction with RNA polymerase. Some activators function by altering the conformation of promoter DNA. Most transcription repressors function by blocking access of RNA polymerase to their target promoter. In some cases, optimal repression depends on multiply bound repressor molecules that interact in complex ways. Many promoters are regulated by more than one transcription factor. A variety of mechanisms whereby a promoter can be regulated by a repressor and an activator, or by two activators, is known.