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