Biology 12

Chapter 8 Protein Synthesis • MHR 275

it is activated by a separate inducer molecule
(cAMP). Because an inducer plays a role in both
forms of control, the lacoperon is known as an
inducible operon. Another operon in the E. coli
genome functions as a repressible operon.
Under normal conditions, an E. colicell produces
the enzymes required to synthesize the amino acid
tryptophan. But if the cell’s medium already contains
tryptophan, it does not make sense for the cell to
expend energy synthesizing the amino acid from
its precursor molecules.

Figure 8.31 on the following page shows the

structure of the trypoperon in E. coli. This operon

contains five different genes as well as a promoter

and an operator sequence. Outside the operon, a

separate gene codes for a repressor protein. Under

normal conditions, the repressor protein does not

bind to the operator region, so transcription usually

takes place. When tryptophan levels in the cell are

very high, however, some tryptophan molecules

will bind to the repressor proteins. Once this

reaction has occurred, the repressor protein is

Plac O Z Y ac

Plac O Z Y ac

RNA polymerase

transcription

cAMP

activator

(CAP)

activator/cAMP

complex

When there is no glucose in the medium,

cAMP accumulates in the cell and binds to

the activator protein, which in turn binds to

the DNA close to the promoter.

A

The presence of the activator causes RNA

polymerase to bind more readily to the

promoter. As more RNA polymerase binds,

the rate of transcription increases.

B

Figure 8.29Positive gene regulation occurs in the lacoperon via cAMP–CAP action.

Figure 8.30Dual control of gene expression in the E. coli lacoperon

cAMP bound

to activator

activator

promoter

repressor remains

bound on operator

repressor remains

bound on operator

repressor

lactose molecule

(inducer)

RNA polymerase

mRNA

beta-g

glucose high,

lactose absent

no transcription

no transcription

abundant transcription

little transcription

or

glucose low,

lactose absent

glucose high,

lactose present

glucose low,

lactose present

B

C

A

If there is no lactose in the medium,

the repressor will remain bound to the

operator and prevent transcription of the

lacgenes regardless of the level of

glucose present.

A

If the medium contains lactose and

abundant glucose, a lactose molecule

(inducer) will bind to the repressor. When

this happens, the repressor will dissociate

from the promoter to allow transcription to

proceed, although at a relatively low rate.

B

If the medium contains lactose but very

little glucose, a lactose molecule (inducer)

will likewise bind to the repressor. The

repressor then dissociates from the

promoter sequence to allow transcription

to take place. Under these low glucose

conditions, however, a cAMP–activator

complex will bind to the DNA. This action

increases the rate at which RNA

polymerase binds to the promoter and

thus boosts the rate of transcription by

50 to 100 times.

C