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Lac
Operon
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 |