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Addressable Bacterial Conjugation

UC Berkeley iGEM 2005

Michael ChenVlad GoldenbergStephen Handley

Melissa LiJonathan Sternberg

Jay SuEddie WangGabriel Wu

Advisors: Professors Adam Arkin and Jay KeaslingGSIs: Jonathan Goler and Justyn Jaworski

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Overview

I. Project GoalII. Overview of Existing TechnologiesII. Initial Design ConsiderationsIII. The Construct and its ImplementationIV. Current StatusV. Future Directions

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Project Goal

To establish specific cell-to-cell communication between two populations

of bacteria

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Project Goal

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Implementation

NEED: To transfer genetic information from one bacteria to another

MEANS: Bacterial Conjugation

NEED: To specifically control who can read the message

MEANS: Riboregulation

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Bacterial Conjugation• Certain bacterial plasmids are classified as having a “fertility factor” i.e. F+

• Cells that have a F+ plasmid can conjugate and transfer their DNA to other bacteria

F+F-

F Pilus Formation

F FF

F+

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Choosing Conjugative Plasmids

There are many plasmids that are classified as conjugative.. For our project, we used F and RP4 plasmids for the following reasons:

•F and RP4 exhibit differing pili lengths, biasing the order in which F and RP4 will conjugate

•F and RP4 do no conjugate with themselves

•F and RP4 are among the most studied and well-characterized conjugative plasmids

•F and RP4 plasmids are readily available

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Important Facts about Conjugative plasmids

• Conjugative plasmids are very large, from 60k – 100k basepairs long

•The TraJ protein is a regulatory protein responsible for initiating the DNA transfer cascade

•DNA transfer during conjugation always begins at a specific sequence on the plasmid, OriT, the Origin of Transfer.

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Modification of conjugative plasmids

• TraJ was cloned and placed into biobrick plasmids under the control of promoters of our choosing

• The OriT region was also cloned and placed into biobrick plasmids thus creating small, mobilizable plasmids

• The OriT region and TraJ gene were knocked out with Lambda-Red mediated recombination to prevent unwanted transfer of the F/R plasmid

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Conjugation Results

• An R-plasmid bearing cell can conjugate with an F-plasmid bearing cell•The F plasmid and R-plasmid knockouts fail to conjugate• The biobricked OriT-R plasmid is mobilizable by the R-plasmid knockout

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The Riboregulator

Isaacs et al., Nature Biotechnology, 2004

• Method of postranscriptional control of gene expression

• cis-repressive sequence (“lock”) upstream of a gene’s coding region forms a hairpin, sequestering the ribosome binding site

• trans-activating (“key”) mRNA strand binds and opens the hairpin thus allowing access to the RBS.

• Highly specific activation occurs. Very similar lock and key pair sequences do not exhibit crosstalk

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Biobricked Riboregulator

Lock from Isaacs Paper

• Tacking biobrick ends onto the end of the lock sequence would be ineffective due to the distance restrictions between a ribosome binding site and a gene’s start codon

• The mixed site was thus incorporated directly downstream of the ribosome binding site

• The five base pair region between the hairpin loop and ribosome binding site was used as our address space to create two new lock sequences

RBS region Biobrick Mixed Site

Predicted mRNA structure of one of our Locks

Address Region Hairpin loop Start of locked gene

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Biobricked Riboregulator

RBS region Biobrick Mixed Site Address Region Hairpin loop Start of locked gene

crR12 locktaR12 key

Lock 1

Key 1

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Biobricked Riboregulator

• Activation by the key sequences was highest when transcribed five nucleotides from the transcription start site (Isaacs, et al.)

• We created a biobricked derivative of the E. Coli rrnb P1 promoter to provide constitutive production of our keys

• Three nucleotides of the biobrick suffix were nested into the 5’ end of the wildtype sequence in order to transcribe the keys at the desired five nucelotide distance.

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Unlocking the Riboregulator

RBS region Biobrick Mixed Site Address Region Hairpin loop Start of locked gene

Key 1

Key 2

Lock 2

Lock 1

RBS now accessible

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Biobricked Riboregulator

Locking Strength Assay

0

50

100

150

200

250

300

350

0 6400 12800 19200 25600 32000 38400 44800 51200 57600 64000 70400 76800

Time (secs)

RFP Activity

rfp

l1

l2

controlConstituitely On RFP

Lock 1

Lock 2

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Riboregulator Construction• Locks and keys are separated at hairpins into pairs of easily ordered oligos ~ 30 bp.

•One of each pair is ordered phosphorylated for easy ligation of annealed products

•Anneal pairs in separate tubes (heat to 95°C, unplug heatblock), combine, ligate.L11 5’- ctagag.aactagaatcacctcttggatttgggt

L12 3’- tc.ttgatcttagtggagaaccta - p L13 5’- p - attaaagaggaga.tactagtagcggccgctgca

L14 3’- aacccataatttctcctct.atgatcatcgccggcg

When annealed and ligated, result already has XbaI and PstI sticky ends…ready for assembly

• Keys require extra pair due to inclusion of key terminator (hairpin) within the part.

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Construction

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Parts Used

J01004

J01005

J01000

J01001

J01002

J01003

J01006

J01008

J01009

J01011

J01010 E0420

i12351

E0840

E0420

I0500

R0040

I12007

C0051

B0034

B0015

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Construction Path

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R-Cell Plasmids

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Sequence of Eventsarabinose

TraJF

F-Cell

R-Cell

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Sequence of Events

F-Cell

R-Cell

cIcI

TraJR

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Sequence of Events

F-Cell R-Cell

spoOA

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Modular Design

•Why didn’t we just lock the fluorescent proteins?

• Modularity and flexibility of design (send out inquiry for message verification!) with the addition of spoOA, cI signal

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Progress thus far…

CFP key2 OriTF lock1 cI GFP

lock2 spo0A YFP

pRM

pspoIIEON

ON ONN ON

RFP key1

ON ONNpRM

TraJROriTR

Non-mobilized plasmid Mobilizable plasmid

Moblizable plasmidNon-mobilized plasmid

F-bearing cell

R-bearing cell

pBAD

TraJF

ara

RBSRBS

RBS RBS RBS

RBS

Unforseen Eco sitesite-mutagenesis time

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Implementation Issues•Fluorescence is sometimes inhibited after conjugation

• Slight leakiness of the lock we designed

• Need to add or knockout an antibiotic resistance to one of the plasmids for selection purposes

• Time-scale of conjugation is slow

•Cloning is a subtle art

Locking Strength Assay

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50

100

150

200

250

300

350

0 6400 12800 19200 25600 32000 38400 44800 51200 57600 64000 70400 76800

Time (secs)

RFP Activity

rfp

l1

l2

control

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Next Steps in Implementation

• Finish constructs

• Test our keys with our locks to observe activation and cross-talk

• Prove that our biobricked OriT-F plasmids is mobilizable

• Create a ‘stop’ message to end communication after the programs are received or a reset method to return the cells to their original state

• Determine the effect of copy number and mobilization

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A Look into…

The Future!

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Application: The Bacterial Network

Because “channels of communication” are limited only by the large number of unique riboswitches, multiple pairs of cell-to-cell communication can occur in a single culture

One can envision a network of celluar strains in the same culture, specializing in different tasks and communicating specifically to necessary related strains

Coordinator Cell

Intermediate A

Reactant A

Reactant B

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Addressable Conjugation vs. Chemically Based Communication

•At its heart, our construct creates two one-way channels of communication

•Quorum sensing exhibits cell-cell communication by using a chemical signal as its message

•One can easily imagine a construct similar to ours with two different chemical carriers. For example, AHL and DHL.

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Addressable Conjugation vs. Chemical Communication: Disadvantages

•Slower•Conjugation ~ 8-18 hours•Chemical Means ~ 2-8 hours

•Conjugation occurs in clumps•Heterogeneity•Limited multiple usage

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Future Projects

•Post-conjugal disengagement

•Multiple “call-and-response”

•Library and characterization of multiple key-lock pairs

•Extending address space

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Addressable Conjugation vs. Chemical Communication: Advantages

•Rational design of separate specific communications channels

•Ability to transfer complex genetic information, instead of a single chemical signal

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Addressable Conjugation Paradigm The Bacterial Internet

Cells that transfer information are web servers

Cells that selectively express the key can be thought as accessing the web page. The key becomes the URL.

Cellular “Server”

User Cell A

User Cell B Selective key expressionRequesting File Download

User Cell C

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Addressable Conjugation Paradigm Function Modularity

We can envision cells with modular components

Different common genetic programs with similar responses can be loaded on-the-fly depending on differing environmental circumstances

CommonHigh-range Module

Lock A

CommonLow-range Module

Lock B

If (High Conc.) Load AElse Load B

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Berkeley iGem would like to thank the following people

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Plasmid and Gene Providers

•Dr. Virginia Waters: RP4/RK2 plasmid

•Dr. Laura Frost: F-Plasmid

•Philip Silverman: pox38 F-Plasmid

•Dr. Farren Isaacs: Lock and Key Sequences

•Mike Cantor: SpoOA and pspoIIE plasmid

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All the members of the Keasling Lab

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Mario

Can clone anything

Jon Dueber

Can clone anything

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Connie

Lambda Red

Doug

Lambda Red

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Professor Adam Arkin &

Professor Jay Keasling

43Questions?

One more animation