The genes that are turned on and the level to which they are expressed is carefully regulated in response to environmental cues. As in eukaryotes, the promoter controls the level at which individual genes are transcribed. Remember, though, that in many instances the mRNAs transcribed from these promoters are polycistronic. This allows the expression of the group of genes downstream of a single promoter to be coordinately controlled. C. Positive control also occurs in E. coli .
Some operons are under both positive and negative regulation.
The lac operon provides an example of this. Remember that, in eukaryotes, a large set of general transcription factors are required for the assembly of the basal transcription apparatus at the core promoter. Regulatory transcription factors modulate activity of the basal transcription apparatus. 1. a DNA binding domain 2. and a second domain that either activates or represses transcription 3. An experiment that clearly demonstrated this is outlined in Fig.