Gene expressions are strictly controlled at many levels to ensure the organism having the appropriate response to its environment or internal changes. This is important for prokaryotes because there are usually single-cell organisms, and they largely depend on their environment for all of their activities.
Transcription in bacteria
In bacteria transcription often occur as polycistrons, i.e., many functional-related genes are clustered and transcribed under the same types of regulation. These are called operons. An operon usually contains regulatory genes and structure genes. The gene expression can be induced under certain circumstances or be constitutive.
Lac operon are activated by lactose, which binds to Lac I, a Repressor, and removes it from the operator sequence, and therefore release the repression from Lac I, the consequence is that the structure genes lac Z. lac Y and Lac A are expressed, and the cells are able to use lactose as carbon source.
Similar to Lac operon, Trp operon has TrpR repressor for normal repression of the operon. But in contrary to lac operon, TrpR is normally inactivate, only in the presence of large amount of Trp, TrpR is activated and the trp operon is suppressed. Trp operon also has an attenuator sequence located within the operon to enable the operon sense the decrease or increase of the trp in the environment.
Gene expression in Bacteriophage
When l phage switch between lysogeny and lytic cycles, two repressors play critical roles: cI and Cro. cI encodes Lambda repressor. Cro encode a protein that controls the repressor (and other genes). When cI proteins predominate, phage remains in the lysogenic state; When Cro proteins predominate, phage enters the lytic state. The regulation on cI and Cro is very artistic and delicate.