Biology Reference
In-Depth Information
FIGURE 1.1
The structure of the
lac
operon. The RNA polymerase binds at the promoter and then
proceeds along the
LacZ
,
LacY
,and
LacA
genes, copying them into a single mRNA which
encodes the three proteins
-galactosidase, lactose permease, and transacetylase. The
operator is a controlling region. See the text for more details.
β
into the cell by lactose permease and then broken down by
-galactosidase. We say
that the operon is on. When all of the lactose is used up, there will be no allolactose,
so the
lac
repressor protein will bind the operator and the synthesis of mRNA will
stop. Levels of lactose permease and
β
β
-galactosidase will fall. The operon has turned
itself off.
If both glucose and lactose are available to
E. coli
, glucose, the preferred energy
source, is always used up before the bacterium begins to utilize the lactose. This
is controlled by the mechanism of
catabolite repression
. Without it, the presence
of lactose would cause allolactose to be made, preventing the
lac
repressor protein
from binding the operator and allowing the transcription of the
lac
operon. In reality,
the availability of glucose represses this process, and when both glucose and lactose
are available, RNApolymerase is not able to initiate transcription sufficiently to ensure
the levels of production of mRNA,
-galactosidase, and lactose permease reached
under the condition of exclusive lactose availability. This failure of transcription
initiation is called catabolite repression.
The mechanism of catabolite repression requires a DNA-binding protein, called
CAP (for
catabolite activator protein
), which is the product of the
crp
(for cAMP
receptor protein) gene. The CAP protein is capable of binding DNA only in the
presence of cAMP. This small molecule is a common cellular signal or messenger
and is produced by the enzyme adenylate cyclase. When glucose is present, adenylate
cyclase does not produce cAMP. When there is no glucose, cAMP is produced. When
cAMP binds CAP, the CAP-cAMP complex attaches to the DNA at the
lac
promoter
and facilitates the attachment of RNA polymerase at the
lac
promoter. If there is
no cAMP present, the CAP protein can't bind to the DNA, the RNA polymerase
β
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