Operons: The Basic Concept

• In bacteria, genes are often clustered into operons, composed of– An operator, an “on-off” switch– A promoter– Genes for metabolic enzymes

• Bacterial cells divide by binary fission– Which is preceded by replication of the bacterial

chromosomeReplicationfork

Origin of replication

Termination of replication

Figure 18.14

Mutation and Genetic Recombination as Sources of Genetic Variation

• Since bacteria can reproduce rapidly– New mutations can quickly increase a population’s

genetic diversity

• An operon– Is usually turned “on”– Can be switched off by a protein called a repressor

• The trp operon: regulated synthesis of repressible enzymes

Figure 18.21a

(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.

Genes of operon

Inactiverepressor

Protein

Operator

Polypeptides that make upenzymes for tryptophan synthesis

Promoter

Regulatorygene

RNA polymerase

Start codon Stop codon

Promoter

trp operon

5

3mRNA 5

trpDtrpE trpC trpB trpAtrpRDNA

mRNA

E D C B A

DNA

mRNA

Protein

Tryptophan(corepressor)

Active repressor

No RNA made

Tryptophan present, repressor active, operon off. As tryptophanaccumulates, it inhibits its own production by activating the repressor protein.

(b)

Figure 18.21b

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

• In a repressible operon– Binding of a specific repressor protein to the

operator shuts off transcription• In an inducible operon– Binding of an inducer to an innately inactive

repressor inactivates the repressor and turns on transcription

• The lac operon: regulated synthesis of inducible enzymes

Figure 18.22a

DNA

mRNA

ProteinActiverepressor

RNApolymerase

NoRNAmade

lacZlacl

Regulatorygene

Operator

Promoter

Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.

(a)

5

3

mRNA 5'

DNA

mRNA

Protein

Allolactose(inducer)

Inactiverepressor

lacl lacz lacY lacA

RNApolymerase

Permease Transacetylase-Galactosidase

5

3

(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.

mRNA 5

lac operon

Figure 18.22b

• Inducible enzymes– Usually function in catabolic pathways

• Repressible enzymes– Usually function in anabolic pathways

• Regulation of both the trp and lac operons– Involves the negative control of genes, because the

operons are switched off by the active form of the repressor protein

Positive Gene Regulation

• Some operons are also subject to positive control– Via a stimulatory activator protein, such as

catabolite activator protein (CAP)

Promoter

Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.

(a)

CAP-binding site OperatorRNApolymerasecan bindand transcribe

InactiveCAP

ActiveCAPcAMP

DNA

Inactive lacrepressor

lacl lacZ

Figure 18.23a

• In E. coli, when glucose, a preferred food source, is scarce– The lac operon is activated by the binding of a

regulatory protein, catabolite activator protein (CAP)

• When glucose levels in an E. coli cell increase– CAP detaches from the lac operon, turning it off

Figure 18.23b(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.

When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.

Inactive lacrepressor

InactiveCAP

DNA

RNApolymerasecan’t bind

Operator

lacl lacZ

CAP-binding site

Promoter