Here we describe the basic features of transcription control in bacteria, using the lac operon in E. coli as our primary example. Many of the same processes, as well as others, are involved in eukaryotic transcription control.

The lac operon encodes three enzymes required for the metabolism of lactose,a sugar present in milk. Since a bacterial operon is transcribed from one start site into a single mRNA, all the genes within an operon are coordinately regulated; that is, they are all activated or repressed to the same extent. Transcription of operons, as well as of isolated genes, is controlled by an interplay between RNA polymerase and specific repressor and activator proteins. In order to initiate transcription, however, E. coli RNA polymerase must be associated with one of a small number of sigma factors, which function as initiation factors. The most common one in bacterial cells is sigma