1 The Operon 操縱元 a functioning unit of genomic material containing a cluster of genes under the control of a single regulatory signal or promoter

1

The Operon 操縱元

a functioning unit of genomic material containing a cluster of genes under the control of a single regulatory signal or promoter

Ex Biochem c12-operon 2

12.1 Introduction

Figure 12.1

Ex Biochem c12-operon 312.2 Regulation Can Be Negative or Positive

In negative regulation, a repressor protein binds to an operator to prevent a gene from being expressed.

Figure 12.2


In positive regulation, a transcription factor is required to bind at the promoter. This enables RNA

polymerase to initiate transcription.

Enhancer, activator

Figure 12.3

12.2 Regulation Can Be Negative or Positive


12.3 Structural Gene Clusters Are Coordinately Controlled Genes coding for proteins that function in the same

pathway may be: located adjacent to one another controlled as a single unit that is transcribed into a

polycistronic mRNA

Figure 12.4

Ex Biochem c12-operon 612.4 The lac Genes Are Controlled by a Repressor

Transcription of the lacZYA gene cluster is controlled by a repressor protein. The repressor binds to an operator that overlaps the

promoter at the start of the cluster. The repressor protein is a tetramer of identical

subunits coded by the gene lacI.

Figure 12.5


12.5 The lac Operon Can Be Induced Small molecules that induce an operon are identical with or

related to the substrate for its enzymes. β-galactosides are the substrates for the enzymes coded by

lacZYA. In the absence of β-galactosides, the lac operon is expressed

only at a very low (basal) level. Addition of specific β-galactosides induces 誘發

transcription of all three genes of the operon. The lac mRNA is extremely unstable;

as a result, induction can be rapidly reversed. The same types of systems that allow substrates to induce

operons coding for metabolic enzymes can be used to allow end-products to repress the operons that code for biosynthetic enzymes.


Figure 12.06: lac expression responds to inducer.

Ex Biochem c12-operon 912.6 Repressor Is Controlled by a Small Molecule Inducer

An inducer functions by converting the repressor protein into a form with lower operator affinity.

Repressor has two binding sites: one for the operator one another for the inducer

Repressor is inactivated by an allosteric interaction: Binding of inducer at its site changes the properties

of the DNA-binding site


Inducer of lac Operon

IPTG: common inducer for lac Operon used in lab

Similar structure to lactose Although can NOT

be digested by beta-galactosidase


Figure 12.07: A repressor tetramer binds the operator to

prevent transcription.

Figure 12.08: Inducer inactivates repressor allowing gene

expression.http://www.youtube.com/watch?v=oBwtxdI1zvk


12.7 cis-Acting Constitutive Mutations Identify the Operator

Mutations in the operator cause constitutive expression of all three lac structural genes.

These mutations are cis-acting and affect only those genes on the contiguous 連續的 stretch of DNA.

Cis-acting Referring to a regulatory sequence in

DNA (e.g., enhancer, promoter) that can control a gene only on the same chromosome.

In bacteria, cis-acting elements adjacent or proximal to the genes they control, whereas in eukaryotes they may also be far away

Figure 12.9


12.8 trans-Acting Mutations Identify the Regulator Gene

Mutations in the lacI gene: are trans-acting affect expression of all lacZYA clusters in the bacterium

trans-acting Referring to DNA sequences encoding diffusible

proteins (e.g., transcription activators and repressors) that control genes on different chromosomes

Mutations that eliminate lacI function: cause constitutive expression are recessive


Mutations in the DNA-binding site of the repressor are constitutive because the repressor cannot bind the operator.

Mutations in the inducer-binding site of the repressor: prevent it from being

inactivated cause uninducibility

Mutations in the promoter are: uninducible cis-acting

Figure 12.10

12.8 trans-Acting Mutations Identify the Regulator Gene


12.14 Repressor Protein Binds to the Operator Repressor protein binds to the double stranded

DNA sequence of the operator. The operator is a palindromic sequence of 26 bp.

Figure 12.17

Operons in eukaryotes

gene order in eukaryotes is NOT random. numerous reports of gene clusters of related

function in eukaryotes, even humans significant tendency for genes from the same

metabolic pathway to cluster. Extensive clustering of non-homologous

genes that are co-ordinately expressed in eukaryotes, including humans


Operons in eukaryotes

At the functional level, physical clustering may be advantageous because it allows groups of genes to be co-ordinately regulated at the levels of nuclear organization and/or chromatin.

The alleles could interact well by being co-localized in regions of chromosomes that facilitate co-ordinate regulation



Hurst, 2004


Osbourn, 2009

Documents

1 The Operon 操縱元 a functioning unit of genomic material containing a cluster of genes under the control of a single regulatory signal or promoter