Activation and repression of transcription initiation in bacteria.

A remarkable feature of bacteria is their ability to respond to environmental stimuli and to adapt to changing environmental conditions. Rapid adaptation is achieved by switching on and off the expression of specific genes. Bacteria are very efficient at increasing the production of certain proteins and enzymes when they are needed, only to shut off their production when it becomes metabolically and energetically wasteful. Tight control of bacterial gene expression can be achieved at many levels, including regulation of transcription initiation, transcript elongation, translation, messenger RNA stability or availability and protein turnover. These mechanisms are not mutually exclusive, and expression of some genes is subject to all the above levels of regulation. In this chapter we focus on regulation at the level of transcription initiation simply because, in most cases, it is the dominant form of regulation. Most of this chapter concerns regulation in the simple enteric bacterium Escherichia coli, which is rightly regarded as a paradigm that can instruct our