Chapter 6

Regulation of Prokaryotic Gene Expression

6.1 Overview

1. What are Gene Expression and Regula

tion?

(1) Gene expression Constitutive Expression Regulated/Adaptive Expression

(2) Gene regulation/control

2. Influencing factors

Nutritional status

Environmental factors

Hormone level

Developmental stage

3. Operon

(1) What is an operon?

(2) Operon types

Inducible operon

Repressible operon

(3) Gene regulation of operon

Negative regulation

Positive regulation

负控诱导系统 正控诱导系统

正控阻遏系统 负控阻遏系统

1. Organization of lac operon

lacI

Regulatory gene

lacZ lacY lacA DNA

mRNA

β -GalactosidasePermease

Transacetylase

Protein

Structural Genes

PlacI PlacOlac

repressor

6.2 The Regulation of lac Operon

3. Positive control of lac operon cAMP CAP (catabolite activator protein) or cAMP receptor protein (CRP)

4. Negative contral of lac operon

p t

pt

lacI lacZ lacY lacA

Repressor -galactosidasetranscripts

Permease Transacetylase

1. + Inducer (lactose/ allolactose)

o

InducedRepressor cannot bind

2. No Inducerp t

lacI lacZ lacY lacA

o

RepressedRepressor binds to laco

transcript

Animation

1. Organization of trp operon

6.3 The Regulation of trp Operon

2. The Repression of trp Operon

trpR → Repressor

Corepressor

Attenuator

3. The Attenuation Mechanism of trp Operon

Mechanism of Attenuation When Ribosome reaches Trp codon

Trp is scarce Trp is aboudant

Attenuation model in Trp starved cells.

Attenuation model in Trp non-starved cells

Animation

5. Generality of Attenuation

1. The Regulation of ara Operon

(1) organization

6.4 The Regulation of Other Operons

P t

Pt

araC araB araA araD

Activator/ Repressor

I1 I2

AraC dimer

CRP

O2 O1

Arabinose absent →Negative control

araB A D

O1 CRP araI

araC

O2

PCPBAD

Blocked

Arabinose present → Positive control

araB A D

O1 CRP araI

araC

PCPBAD

O2

Transcription

2. The Regulation of rRNA Operon

(1) organization

6.5 Riboswitches

1. What is a riboswitch?

Riboswitches have two important domains:

an aptamer

an expression platform

2. Mechanism of Riboswitches

(1) Inhibition of Translation Initiation

(2) Transcription termination

(3) Auto-cleavage

3. Tempting Targets

4. Riboswitches and the RNA World hypothesis

5. Riboswitches as Antibiotic Targets

Blount KF, Breaker RR. Riboswitches as antibacterial drug targets". Nat Biotechnol, 2006, 24 (12): 1558-64.

Brantl, S. Bacterial gene regulation: from transcription attenuation to riboswitches and ribozymes. Trends Microbiol, 2004, 12:473-475.

Mandal, M. and R. R. Breaker. Gene Regulation by Riboswitches. Nat Rev Mol Cell Biol, 2004, 5:451-463.

6.6 Time Regulation

Different sigma factors recognize different promoters and thus, the availability of sigma factors can regulate the transcription of genes associated with these promoters. 

1. Time Regulation of Sigma Factors

Early

Middle

Late

Uncommitted.

2. Time Regulation of λ phage

Committed.

Lysogeny

Lysis

Execution

Lysogeny

Lysis

1. Translation Initiation of Regulation

(1) Ribosome binding site (rbs)

(2) SD sequencing

(3) mRNA secondary structure

(4) Initiational codon

2. Regulation of Rare Codon

3. Transcriptional Regulation of Overlapping Gene

4. Translational Repression

6.7 Post-Transcriptional Regulation

Summary1. Concepts

Gene Expression Operon Attenuator

Constitutive Expression

Inducible Operon Attenuation

Regulated Expression Repressible Operon Riboswitch

Gene Regulation

2. Differences between inducible operon and repressible operon3. The regulation mechanism of lac operon4. The regulation mechanism of trp operon (attenuation)5. The regulation mechanism of ara operon6. The regulation mechanism of riboswitches