Download Aim: What is positive feedback of bacterial operons?

Aim: What is positive feedback of
bacterial operons?
Operon summary
 Repressible enzymes generally function in anabolic
pathways, synthesizing end products. (tryptophan
synthesis).
 Inducible enzymes usually function in catabolic
pathways, digesting nutrients to simpler molecules.
(lactose metabolism).
 Both repressible and inducible operons demonstrate
negative control because active repressors can only
have negative effects on transcription.
Positive Gene Control
 Positive gene control
occurs when an activator
molecule interacts directly
with the genome to switch
transcription on.
 The activator site is next to
the promoter site and binds
to transcription factors
which help RNA
polymerase bind more
easily to the promoter.
 This increases the rate of
transcription of the operon.
An Example of positive gene
control (lac operon)
 1) cAMP receptor protein
(CRP) is a transcription
factor that is normally
inactive.
 2) When glucose
concentrations are low, a
messenger substance,
cAMP, is released.
 3) cAMP binds to CRP and
activates it.
An Example of positive gene
control (2)
 4) active CRP binds to
the activator site and
helps RNA polymerase
attach to the promoter
more effectively.
 5) Lactose is
metabolized more
quickly.
The cell would rather use glucose !!
 If glucose levels are
sufficient and cAMP levels
are low (lots of ATP), then
the CRP protein has an
inactive shape and cannot
bind upstream of the lac
promotor.
 The lac operon will
be transcribed but
at a low level.
Summary
Lac operons use both negative control
(repressors) and positive control (CRP)
to accomplish lactose metabolism.