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Chapter 13Microbial genetics 2:

Regulation of gene expression

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2

Lecture overview

Objective: To gain an understanding of the mechanisms by

which bacteria regulate gene expression

Outline:

I. Regulation of transcription

II. Global regulatory systems

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Overview

regulation of gene expression

conserves energy / raw materials

maintains homeostasis

adaption to envir. changes

multiple levels

transcription*

translation

post-translation

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PART I:

Regulation of transcription

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Fig. 13.4

A. Introduction

control of enzyme expression key

want activity only when needed

categories

constitutive

inducible

repressible

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A. Introduction

control of enzyme expression key

want activity only when needed

scenarios for control of enzyme expression

induction: presence ofsubstrate incr. enzyme expression

e.g. many catabolic enzymes = inducible

repression: presence ofend productdecr. enzyme expression

e.g. many biosynth. enzymes = repressible

control of expression: via regulatory proteins

negative transcriptional control: initiation inhibited

repressor proteins

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B. Negative transcriptional control (Fig. 13.3)

1. of an inducible enzyme: repressoractive w/o inducer

e.g. catabolic enzyme not made w/o substrate

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2. of a repressible enzyme: repressor requires corepressor for activity

e.g. biosynthetic enzyme not made when end product present

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B1. Negative transcriptional control; inducible genes

e.g. lacoperon (Fig. 13.5)

E.coli; can use lactose as C source

3 catabolic enzymes; metabolize lactose

components

LacI = repressor

operator: binds repressor

operon

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=>turn on genes in presence of lactose (i.e. inducible)

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no lactose: LacI expressed and active; binds operator

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lactose present: LacI expressed but inactive

allolactose binds LacI repressor; inactivates

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B2. Negative transcriptional control; repressible genes

e.g. trp operon (Fig. 13.8)

E.coli; tryptophan synthesis

5 biosynthetic enzymes

when Trp abundant, dont make more!!

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Trp low: Trp repressorinactive

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Trp abundant: Trp repressoractive

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C. The Lac operon and catabolite repression

E.coli in glucose+lactose medium

glucose first, then lactose

diauxic growth: biphasic

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C. The Lac operon and catabolite repression

not fully understood; catabolite repression important

glucose catabolic enzymes constitutive

other C sources needing processing (e.g. lactose): regulated

=> catabolite repression

regulator: catabolite activator protein (CAP)

CAP required for expression

binds CAP binding site; allows RNA pol to bind

active: cAMP-bound

glucose absent: cAMP high

e.g. catabolite repression oflacoperon

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C. The Lac operon and catabolite repression (Fig. 13.20)

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D. Other mechanisms of transcriptional regulation

attenuation

halting of transcription elongation prior to termination

formation of transcriptional pause / termination loops

based on metabolite availability

e.g. trp operon

riboswitches

form of attenuation

differential folding of mRNA leader region: affects RNA polymerase

activity

folding due to availability of effector molecule

e.g. metabolite availability

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PART II:

Global regulatory systems

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Introduction

affect many genes / pathways simultaneously

more than accomodated by 1 operon

why?

complex responses

differential operon regulation in single response

terms

regulon

genes / operons controlled by same reg. protein

assoc. w/ common function

e.g. heat shock

modulon

regulon w/ operons also controlled separately

e.g. catabolite repression

stimulon

regulon(s) / modulons respond together to environ. stimulus

e.g. phosphate limitation response 19

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A. Mechanisms of global regulation

1. alternate sigma factors

different sigma factors =>

different promoters

diffs in -35 / -10 regions

substitution of sigma factors

changes gene expression of many

genes and operons example: E. colisigma factors

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B. Global regulation: 1) B.subtilis sporulation

multiple control mechanisms

transcriptional initiation; phosphorelay; alternate sigma factors

vegetative growth: WA / WH

transcr. of normal survival genes

starvation: expr. of alt. sigma fators

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Fig. 13.27

B. Global regulation: 1) B.subtilis sporulation

sensor kinase:KinA

autophosphorylates in response to envir. signals (e.g. starvation)

response regulator: Spo0A

Spo0A-P: active transcription regulator

activates WF production / sporulation

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B. Global regulation: 2) Chemotaxis in E.coli

controls chemotactic response: flagellar rotation

chemoreceptors: methyl-accepting chemotaxis proteins (MCP)

bind chemoattractant

stim. ccw rotation

MCP dimer bound to

CheW + CheA

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Fig. 13.15

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sensor kinase: CheA

no attractant bound to MCP: CheA autophosphorylates phosphorylates CheY

response regulator: CheY

CheY-P => FliM; clockwise rotation; tumble

CheZ: dephos. CheY (~10s)

attractant bound to MCP: CheA autphos. inhibited

ccw motion allowed; run

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Fig. 13.15

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absence of chemoattractant: random movement

runs and tumbles

CheY phos. / dephos

chemoattractant present: directional movement lowering the frequency of tumbles

ccw rotation

dephos. of CheY (no CheA phos)

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B. Global regulation: 3) Quorum sensing in V.fischeri

bioluminescence @high density

intercellular communication

signal: N-acyl homoserine lactone (AHL)

AHL synthase (luxIgene)

positively autoregulated

LuxR: transcr. activator

requires AHL

http://www.nsf.gov/news/mmg/

media/images/vibrio_f1.jpg27

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B. Global regulation: 3) Quorum sensing in V.fischeri

signal: N-acyl homoserine lactone (AHL)

diffuses out; accumulates; diffuses back into cell

activates LuxR

activates luxI; luxCDABEG

mechanism: autoinduction

AHL = autoinducer

Fig. 13.25

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Chapter 13 summary

I. Regulation of transcription

A. Intro

B. Negative transcr. control

1. lacoperon (inducible)

2. trp operon (repressible)

C. Catabolite repression

D. Other mechanisms(attenuation; riboswitches)

II. Global regulatory systems

A. Mechanisms of globalregulation

alt. sigma factors

phosphorelay

B. Examples

1. B.subtilis sporulation2. E.colichemotaxis

3. V.fischeriquorum

sensing

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13 BI323 Genetics 2