Lecture # 40 Dr. Buckhaults

Lecture # 40 Dr. Buckhaults

Genetic Regulatory Mechanisms

Control of Gene Expression • Transcriptional control • Clustering of genes with related function • Coordinate control of genes with related function • Polycistronic mRNA

Inducible Genes - Operon Model • Definition: Genes whose expression is turned on by the presence of some substance – Lactose induces expression of the lac genes – An antibiotic induces the expression of a resistance gene

• Catabolic pathways

Lactose Operon • Structural genes – lac z, lac y, & lac a – Promoter – Polycistronic mRNA

• Regulatory gene – Repressor

• Operator • Operon • Inducer - lactose

Regulatory Gene

i

Operon p

o

z

y

a

DNA m-RNA Protein

β -Galactosidase Transacetylase Permease

Lactose Operon • Inducer -- lactose – Absence

Absence of lactose i

p

y

a

No lac mRNA

– Presence

• Negative control

z

Active

• Active repressor • No expression • Inactivation of repressor • Expression

o

Presence of lactose i

p

o

z

y

a

Inactive β -GalactosidasePermease Transacetylase

• Definition: Control of an operon by glucose • Catabolic operons

Units of -galactosidase

Catabolite Repression (Glucose Effect) - glucose

Glucose added + glucose

Time (hr) + lactose

Mechanism of Catabolite Repression • c-AMP • CAP (CRP) protein • CAP-cAMP complex – Promoter activation

• Positive control

Absence of glucose

Adenyl cyclase c-AMP ATP

CAP

i

p

o

z

y

a

Active

Inactive β -GalactosidasePermease Transacetylase

Maximum expression

Mechanism of Catabolite Repression • Glucose↑:cAMP↓ • CAP (CRP) protein • No CAP-cAMP complex – No Promoter activation

Presence of glucose

Adenyl cyclase ATP

X

CAP

i

p

o

z

y

a

Inactive β -GalactosidasePermease Transacetylase

Low level expression

Repressible Genes - Operon Model • Definition: Genes whose expression is turned off by the presence of some substance (co-repressor) – Tryptophan represses the trp genes

• Biosynthetic pathways – Co-repressor is typically the end product of the pathway

Tryptophan Operon • Structural genes – trp E, trpD, trpC trpB & trpA – Common promoter • Regulatory Gene – Apo-Repressor

Regulatory Gene

R

Operon P

O

L

E

D

C

• Inactive

• • • •

Operator Leader Operon Co-repressor – Tryptophan

Inactive repressor (apo-repressor)

5 Proteins

B

A

Tryptophan Operon Absence of Tryptophan

• Co-repressor -tryptophan

R

P

O

L

E

D

C

B

A

B

A

– Absence of tryptophan • Gene expression

– Presence of tryptophan

Inactive repressor (apo-repressor)

• Activates repressor • No gene expression

5 Proteins

Presence of Tryptophan

Negative control •

Role of tryptophan •

R

P

O

L

E

D

C

No trp mRNA Inactive repressor (apo-repressor)

Trp (co-repressor)

Attenuation • Definition: Premature termination of transcription

L P

• Leader region – – – –

Leader transcript Translation start Translation stop Tryptophan codons

O

1

2

3

4

2

3

4

E

DNA

UGA

AUG 1

2 trp codons

RNA

4 3

2

• Mutually exclusive mRNA secondary structure

1

Attenuation

UUUUUUU

– region 1 : region 2 – region 2 : region 3 – region 3 : region 4 2

3

• Coupled transcription and translation

4 1

UUUUUUU

Attenuation High tryptophanyl-t-RNA

Low tryptophanyl-t-RNA UGA

AUG UGA

AUG

1 1

3

2

3

2

4

4

2 trp codons

2 trp codons

2

3

3

4

UUUUUUU 1

2

Attenuation

4

UUUUUUU

1

No Attenuation

Regulation of Enzyme Activity • Feed back inhibition • Epigenetic modification – Post translational modifications • Phosphorylation/dephosphorylation • Adenylation/deadenylation