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Section H Host and Vector H1 E.coli /Plasmid Vectors H2 E.coli /Bacterophage Vectors H3 Yeast/YAC and E.coli /BAC H4 Eukaryotic Host/Vectors. H1 E.coli /Plasmid Vectors. E.coli /pBR322 plasmid

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H1 e coli plasmid vectors

Section H Host and Vector H1 E.coli/Plasmid Vectors H2 E.coli/Bacterophage Vectors H3 Yeast/YAC and E.coli/BAC H4 Eukaryotic Host/Vectors

Yang Xu, College of Life Sciences

H1 e coli plasmid vectors
H1 E.coli/Plasmid Vectors

  • E.coli/pBR322 plasmid

  • E.coli/pUC plasmid vectors

  • Multiple cloning sites

  • E.coli/pGEM

  • E.coli/T7 Expression vectors

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Ligation products




Ligation products

Ligation products:

  • Recombinant plasmid: With a target fragment.

  • Recreated vectors: When ligating a target fragment into a plasmid vector, the most frequent unwanted product is the recreated vector plasmid

    Screening of ligation products:

  • Agarose gel electrophoresis: For mini-preparations from a number of transformed colonies. Screening by digestion and agarose gel electrophoresis;

  • Specially developed vectors: For large scale preparations. Now more efficient methods based on specially developed vectors have been devised (see below).

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E coli pbr322 plasmid
















E.coli/pBR322 plasmid

Mechanisms--Insertional inactivation of the resistance genes: If a target DNA fragment is ligated into the coding region of tet A, the gene will become insertionally inactivated.



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Twin antibiotic resistance screening



Ampicillin only

Ampicillin and tetracycline

Twin antibiotic resistance screening

1. Transformant plating:

  • Recombinant: can only grow in ampicillin plates;

  • Recreated vectors: can grow in ampicillin and tetracycline plates

    2. Replica plating: The colonies grown on a normal ampicillin plate are transferred, using an absorbent pad, to a second plate containing tetracycline.


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Blue white screening

lac promoter






ampr/X-gal plate

Blue-white screening

  • Example--pUC18 plasmid: This one contains an ampr and a lac Z gene, which encodes the -galactosidase, and is under the control of the lac promoter.

Blue: no insert

White: insert

  • Mechanisms--Insertional inactivation of the lac Z gene:

    • Under the effect of -galactosidase, the substrate X-gal will produce a blue product.

    • 1. The blue colonies: probably contain recreated vector.

    • 2. The white colonies: have no expressed -galactosidase and are hence likely to contain the inserted target fragment.

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Multiple cloning sites


AccI HincII




Multiple cloning sites

  • The first vectors which used blue-white selection also pioneered the application of multiple cloning site (MCS).

    Definition: The pUC series contain an engineered lacZ‘ gene, which has multiple restriction enzyme sites within the first part of the coding region of the gene, which is known as “MCS”.

    Function: The insertion of target DNA in any of these sites, will inactivates the lac Z’ gene, to give a white colony.

EcoRI SacI KpnI BamHI XbaISalI PstI SphI HindIII

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E coli t7 expression vectors









E.coli/T7 expression vectors

  • Definition of expression vectors:

    Cloned geneexpression vector hostfusion protein.

  • Structure

    • T7 promoter: a strong promoter;

    • RBS: ribosomebinding site;

    • ATG: translation initiation condon

    • MCS: Multiple cloning sites

    • TT: transcription terminator.

    • ampr,. ori,

  • His-tag: Some expression vectors are designed to have six histidine codons that encode a hexahistidine tag at the N terminus of the expressed protein, which allows one-step purification on an affinity column containing Ni2+.

Yang Xu, College of Life Sciences

H2 bacterophage vectors
H2 Bacterophage Vectors

  • Bacteriophage 

  • E.coli/ Replacement vectors

  • E.coli/Cosmid vectors

  • E.coli/M13 phage vectors

  • E.coli/pBluescript vectors

  • Hybrid plasmid-M13 vectors

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Bacteriophage l life cycle
Bacteriophage l(life cycle)

Process of phage  infecting E. coli: In brief,

1. Phage injects its linear DNA into E.coli, then ligates into a circle.

2. The circle DNA may replicate to form many “phage particles”,

3. which  are released from the cell by lysis and cell death (lytic phase),  or integrate into the host genome (lysogenic phase).

Lytic life



UV induce

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Coat protein



Liner DNA

Phage 

Cos end

Bacteriophage 



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E coli replacement vectors



 20kb


Short arm


Long arm

48.5 kb

E.coli/ Replacement vectors

Examples: EMBL3 and  DASH.

A representative scheme for cloning:

1. The vector DNA is cleaved with BamH1 and the long (19 kb) and short (9 kb) arms (p116 Fig. 1) are purified;

2. The target fragments are prepared by digestion, also with BamH1 or a compatible enzyme (Sau3A);

3. The target fragments are treated with alkaline phosphatase to prevent them ligating to each other;

4. The  arms and the target fragments are ligated together at relatively high concentration to form long linear products.



Can not


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Packaging and infection

Replication concata-mers


in vitro

Infection of E. coli

A mixture of phage coat proteins and

the phage DNA-processing enzymes

109 recombinants per

mg of vector DNA.



Packaging and infection

The Recombinants that can not be packaged:

1. Ligated  ends which do not contain an insert;

2. The insert is much smaller or larger than the 20 kb;

3. The recombinants with two left or right arms.


in vivo


individual  genomes


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Formation of plaques

E.coli lawn


Formation of plaques

Plaques are the analogs of single bacterial colonies.


The infected E.coli cells from a packaging reaction are spread on an agar plate,

The plate has been pre-spread with uninfected cells, which will grow to form a continuous lawn.

After incubation, phage-infected cells result in clear areas, that are plaques, where cycles of lysis and re-infection have prevented the cells from growing.

  • Recombinant  DNA may be purified:

  • from phage particles isolated from plaques or

  • from the supernatant of a culture infected with a specific recombinant plaque.

Yang Xu, College of Life Sciences

E coli cosmid vectors









(32- 47kb)

37-52 kb

E.coli/Cosmid vectors

  • Structure:

    1. a plasmid origin of replication (ori);

    2. a selectable marker, for example ampr;

    3. a cos site, for re-circulating;

    4. a suitable restriction site for cloning .

Packaging into  phage

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Bacteriophage m13




Bacteriophage M13

Infection: M13 particles attach specifically to E.coli sex pili (encoded by a plasmid called F factor), through a minor coat protein (g3p). Binding of g3p induces a structural change in the major capsid protein. This causes the whole particle to shorten, injecting the viral DNA into the host cell.

Genome features: Size is small (6.7 kb); Single-stranded; Circular genome; DNA; Positive-sense.








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E coli m13 phage vectors
E.coli/M13 phage vectors

Structure:  The phage particles containa 6.7 kb circular ssDNA.  After infection of a sensitive E. coli host, the complementary strand is synthesized, like a plasmid, and the DNA replicated as a dsDNA, the replicative form (RF).

Features:  The host cells can continue to grow slowly.

  • ssDNA: The single-stranded forms are continuously packaged and released from the cells as new phage particles. ssDNA has a number of applications, including  DNA sequencing and  site-directed mutagenesis.

  • dsDNA: The RF (dsDNA) can be purified in vitro and manipulated exactly like a plasmid.

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Cloning in m13
Cloning in M13

Purpose: When the single-stranded DNA of a fragment is required, a M 13 vector can be used as a common cloning tool.

Preparation of ssDNA:

1. Cloning: standard plasmid cloning method can be used to incorporate recombinant DNA into M13 vectors;

2. Transformation: the M13 then infects sensitive E. coli cells;

3. Plating: the host cells grow to form the plaques;

4. Isolation: the ssDNA may then be isolated from phage particles in the growth medium of the plate.

Screening: Blue-white screening using MCSs and lacZ' has been engineered into M13 vectors.

Examples: The M13mpl8 and M13mp19, which are a pair of vectors in which the MCS are in opposite orientations relative to the M13 origin of replication.

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Hybrid plasmid m13 vectors
Hybrid plasmid-M13 vectors

Definition: A number of small plasmid vectors, for example pBlue-script, have been developed to incorporate M13 functionality.

Structure: They contain both plasmid and M13 origins of replication, but do not possess the genes required for the full phage life cycle.

Working ways:

1. Plasmid way: they normally propagate as true plasmids, and have the advantages of rapid growth and easy manipulation of plasmid vectors;

2. Phage way: they can be induced to produce single-stranded phage particles by co-infection with a fully functional helper phage, which provides the gene products required for single-strand production and packaging.

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H3 yac and bac
H3 YAC and BAC

  • Cloning large DNA fragments

  • YAC vectors

  • BAC vectors

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Cloning large dna fragments
Cloning large DNA fragments

  • Problems:

    1. The analysis of genome organization and the identification of genes, particularly in organisms with large genome sizes (human DNA is 3  109 bp, for example) is difficult to use plasmid and bacteriophage  vectors, since the relatively small size capacity of these vectors for cloned DNA means that an enormous number of clones would be required to represent the whole genome in a DNA library.

    2. In addition, the very large size of some eukaryotic genes, due to their large intron sequences, means that an entire gene may not fit on a single cloned fragment.

  • Solution: Vectors with much larger size capacity have been developed to solve these problems.

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Yeast yac vectors







Yeast/YAC vectors

CEN4 is the centromere of chromosome 4 of Yeast. The centromere will segregate the daughter chromosomes.

ARS is autonomously replicating sequence, its function is as a yeast origin of replication.

TRP1 and URA3 are yeast selectable markers, one for each end, to ensure the right reconstituted YACs survive in the yeast cells.

TEL is the telomeric DNA sequence, which is extended by the telomerase enzyme inside the yeast cell.

SUP4 is a gene, which is insertionally inactivated, for a red-white color test, like blue-white screening in E. coli.

Function: YAC vectors can accept genomic DNA fragments of more than 1 Mb, and hence can be used to clone entire human genes.

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E coli bac vectors
E.coli/BAC vectors

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H4 other eukaryotic vectors
H4 Other Eukaryotic Vectors

  • Cloning in eukaryotes

  • Transfection of eukaryotic cells

  • Shuttle vectors

  • Yeast/episomal plasmids

  • Agrobacterium tumefaciens/Ti plasmid

  • Insect cell/Baculovirus

  • Mammalian cell/viral vectors

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Cloning in eukaryotes
Cloning in eukaryotes


  • E. coli as host: Many eukaryotic genes and their control sequences have been isolated and analyzed using gene cloning techniques based on E. coli as host.

  • Eukaryotic Vectors: However, many applications of genetic engineering (see Section J) require vectors for the expression of foreign genes in different eukaryotic species, for example:

    1. Large-scale production of eukaryotic proteins;

    2. Engineering of new plants;

    3. Gene therapy for human.

  • Such kinds of vectors designed for a variety of hosts are discussed in this topic.

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Transfection of eukaryotic cells
Transfection of eukaryotic cells

Problem: The transfection of DNA into eukaryotic cells is more problematic than E.coli transformation, and efficiency of the process is much lower.

Reasons and solutions:

  • In yeast and plant cells, the cell wall must be digested, which may then take up DNA easily.

  • Animal cells in culture take up DNA at low efficiency. If it is treated on their surface with calcium phosphate, the efficiency may be increased.

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Transfection of eukaryotic cells1
Transfection of eukaryotic cells

Other transfection techniques:

  • Electro-poration: By treatment of the cells with a high voltage, which opens pores in the cell membrane.

  • Micro-injection: foreign DNA may be microinjected into cells, by using very fine glass pipettes.

  • Micro-projectiles: DNA may be introduced by micro-projectiles which fire metallic coated with DNA at the target cells.

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Shuttle vectors



Shuttle vectors

Definition: They are the vectors that can shuttle between more than one host, for example, one is E. coli and the other is yeast.

Structure and function: Most of the vectors for use in eukaryotic cells are constructed as shuttle vectors.

  • InE. coli:

    • This means that they can survive and have the genes (ori and ampr ) required for replication and selection in E. coli.

  • In the desired eukaryotic cells:

    • They can also survive in the desired host cells, and let the target insert sequences take effects.

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Yeast episomal plasmids




2 origin


Yeast episomal plasmids

Structure of YEps

a ori: for replication in E.coli

a ampr: for selection in E. coli

a 2 origin: for replication in yest

LEU2: is homologous gene and a selectable marker in yeast, involved in leucine synthesis.

X gene: a shuttle sequence.

X gene

  • Function of YEps

  • It replicates as plasmids

  • It integrates into a yeast chromosome by homologous recombination.

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Agrobacterium tumefaciens ti plasmid i


Agrobacterium tumefaciens/Ti plasmid-I

Definition: Ti plasmid is a kind of plasmid which commonly used to transfer foreign genes into a number of plant species.

plant DNA



  • Function: The bacterium A. tumefaciens can infects and transfer foreign genes into:

    • 1. Dicot plants: tomato, tobacco;

    • 2. Monocot plants, for example rice.

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H1 e coli plasmid vectors

Ti plasmid


Ti plasmid





In E.coli

Improving: Disarmed T-DNA shuttle vectors

  • The recombinant T-DNA can be constructed in a E. coli plasmid;

  • Then transform into the A. tumefaciens cell carrying a modified Ti plasmid without T-DNA.

  • Infecting plant cell culture with A. Tumefaciens.

  • Plating transformed clones.

  • Regenerate plant using hormone

In A. tumefaciens

  • Advantage:

    • Integrate cloned genes easily, and

    • The recombinant plants can be reconstituted from the transformed cells.

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Insect cell baculovirus
Insect cell/Baculovirus

Definition: Baculovirus is an  insect virus which can be used for the overexpression of animal proteins in insect cell culture.


  • Viral promoter: This viral gene has an extremely active promoter.

  • Insect cell culture: The same promoter can be used to drive the over-expression of a foreign gene engineered into the baculovirus genome.

    Function: This method is being used increasingly for large-scale culture of proteins of animal origin, since the insect cells can produce many of the post-translational modifications of animal proteins, which a bacterial expression system cannot.

Baculovirus-infected SF21 cells

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Mammalian cell viral vectors
Mammalian cell/viral vectors

  • SV40: This virus can infect a number of mammalian species. The SV40 genome is only 5.2 kb in size.

  • Since it has packaging constraints similar to phage , so it can be not used for transferring large fragments.

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Mammalian cell viral vectors1
Mammalian cell/viral vectors

  • Retroviruses: They have a ssRNA genome, which is copied into dsDNA after infection. The DNA is then stably integrated into the host genome by a transposition mechanism. They have some strong promoters, and they have been considered as vectors for gene therapy (see Topic J6), since the foreign DNA will be incorporated into the host genome in a stable manner.

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H1 e coli plasmid vectors

That’s all for Section H

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