Chapter 18 Regulation of Gene Expression
18.1 Bacteria often respond to environmental change by regulating transcription
First, cells can adjust the activity of enzymes already present. (feedback inhibition).
Second, cells can adjust the production level or certain enzymes.
The basic mechanism for this control of gene expression in bacteria, described as the
, was discovered in 1961 by Francois Jacob and Jacques Monod.
Operons: The Basic Concept
The switch is a segment of DNA called an
It’s positioned within the
promoter or between the promoter and the enzyme-coding genes, the
operator controls the access of RNA polymerase to the genes.
is the operator, promoter, and the genes they control.
The operator can be switched off by the protein called the
binfs to the operator and blocks attachment of RNA polymerase to the
promoter, preventing transcription of the genes.
repressor is the product of a
, which is
located some distance from the operon it controls and has its own promoter.
operon is not switched off permanently because the binding of
repressors to operators is reversible and the
repressor, like most
regulatory proteins, is an allosteric protein, with two alternative shapes,
active and inactive.
repressor is synthesized in an inactive form with little affinity for the
Tryptophan functions in this system as a
, a small molecule that
cooperates with a repressor protein to switch an operon off.
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
operon is said to be a
because its transcription is
usually on but can be inhibited when specific small molecule binds
allosterically to a regulatory protein.
is usally off but can be stimulated when a specific small
molecule interacts with a regulatory protein.
The gene for B-galactosidase is part of the
operon, which includes two
other genes coding for enzymes that function in lactose utlization.
The regulatory gene,
located outside the operon, codes for an allosteric
repressor protein that can switch off the
operon by binding to the
operon, is active by itself, binding to the operator and switching the
operon off. In this case, a specific small molecule, called an
are induced by a chemical signal while
generally function in anabolic pathways, which synthesize essential
end products from raw materials.
Gene regulation is only positive when a regulatory protein interacts directly
with the genome to switch transcription on.