Negative - accomplished by processes known as induction
and repression (restricted to genes for groups of enzymes
that function together...operons)
induction is used to regulate catabolic pathways and
is accomplished by:
binding of an inducer (generally the substrate
for the enzyme) to a repressor (a specific
protein)
inhibits repressor binding to the operator (the
region between the promoter and the start site)
RNA polymerase now binds to the promoter, thus
allowing it to initiate gene transcription
repression is frequently used to regulate
biosynthetic pathways and is accomplished by:
repressor-corepressor complex binds to the
operator (a corepressor is a molecule related to the
products of the enzyme whose gene is being regulated)
RNA polymerase progression to the start site is
inhibited
gene transcription is inhibited by blocking the
interaction of RNA polymerase with the promoter
Positive - accomplished by activation (restricted to
genes for groups of enzymes that function together ...
operons)
inducer (generally the substrate) binds to an
activator protein
activator now binds to the activator site located
near the promoter
activator interacts with RNA polymerase
RNA polymerase binds the promoter, thus initiating
gene transcription
Attenuation
Attenuation used to regulate biosynthetic pathways (especially
those for amino acids essential metabolites that need precise level
control) and is accomplished as follows:
Abundance of the biosynthetic product leads to
transcription and translation of a leader sequence which contains
an attenuator region
Cleavage of the nascent protein generates an attenuator
peptide
Attenuator peptide interacts with mRNA to induce
formation of a stem-loop structure
Stem-loop structure slows down (attenuates)
transcription of the gene(s) in the operon
Global Regulation
Global regulation is simultaneous regulation of a large number of
unrelated genes or gene products
Catabolite repression
Mechanism
Cyclic AMP (cAMP) binds to an activator
protein
Activator protein binds to a DNA activator
site
RNA polymerase binds to the DNA activator
site
Transcription of the relevant genes occurs
Glucose regulation
when glucose is present in the growth medium, the genes
needed for catabolism of other sugars are generally not
transcribed
this is due to the fact that glucose represses cAMP
synthesis (so activation cannot occur)
Proteases - degradation of enzymes by proteases
can also regulate enzymatic activity
Chaperonins &endash; enhance stability of
proteins by "encouraging" them to fold into a stable, active
conformation; synthesized more during stress
