Team:Edinburgh/Phage Reactors 2.0

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This is Version 2.0 of the Phage Reactor proposal. Version 1.0 is here.

Contents

Overview

There exist situations where:

  • several enzymes are needed to produce the desired product
  • these enzymes work synergistically
  • these enzymes must work outside the cell
This is what we want to build: phage with enzymes (coloured circles) fused to the protein coat.

This proposal allows for the construction of scaffolds containing several enzymes, as well as ensuring that this complex is exported from the cell. This is accomplished by using phage as the scaffold, and attaching enzymes to it by protein fusion.

There are probably many uses of an external reaction scaffold, so this technique is fairly general and could hopefully be used for many purposes, just by swapping in the correct BioBricks. One example is a cellulosome, which is a complex of cellulolytic enzymes that can be used to turn cellulose (tough plant material) into sugar, which can then be fermented into fuel.

Technical notes

The project would probably use phage M13, which is non-lytic (does not kill the bacteria). The major coat protein (p8 aka pVIII) already exists in the Registry as <partinfo>BBa_M13008</partinfo>, although DNA is not available. The entire genome is known (e.g. here is the entire M13KO7 genome) so primers can be designed. New England Bioworks claims:

"The major coat protein pVIII is present at ~2700 copies per virion, of which ~10% can be reliably fused to peptides or proteins."

Phage itself can be ordered if needed, e.g. here.

We will infect our E. coli with (more or less) wildtype phage. Our E. coli will have a plasmid coding for a fusion between an enzyme and pVIII. Proteins can be fused to pVIII at its amino terminal (i.e. 5' end in the DNA), according to Weiss and Sidhu, 2000. pVIII has a leader peptide that is cleaved out, slightly complicating fusion design.

To attach several different proteins to the "reactor", different fusions can be created and all of them expressed on a plasmid. It may be simplest if this is a different plasmid from the phagemid, though plasmid compatibility will have to be kept in mind.

We will need to tune expression levels of the fusion versus the wildtype protein.

Six M13 phage attached to a bead. Each phage could have different enzymes attached.

Genetic instability

Chris notes that the presence of repeated sequences on a plasmid can lead to genetic instability, but this will not be a problem in lab strains, which lack an important recombination enzyme. As for the use of this technology in industry, it will be possible to overcome this problem simply by synthesising coding sequences with as many altered (but synonymous) codons as possible. The (non-iGEM?) group MIT 20.109 Spring07 seem to have thought along these lines, e.g. compare <partinfo>BBa_M13008</partinfo> with <partinfo>BBa_M31281</partinfo>.

Maurice suggests a different approach: make several strains of bacteria each producing phage with just one type of pVIII-fusion. But also make each phage have a pIII-fusion with a protein which could bind some sort of bead. The bead now becomes the complete "reactor". Since each bacteria codes for only one pVIII fusion, there is no repeated sequence problem.

Problems

The question is how efficiently fusions to pVIII can get onto the phage. There are some dire warnings in the literature:

  • In our experience, most large proteins display well below one copy per phage particle. - Sidhu et al (2000)
  • A large 20 kDa protein (human growth hormone, hGH) is not displayed at detectable levels. - Weiss and Sidhu (2000)
  • The properties of the pIV channel may be one of the factors that limit the size of polypeptides that can be displayed on pVIII. - Karlsson (2004)

However, Maurice seemed quite upbeat about the prospects when we met him.

Testing

As proof of concept (i.e. something we can accomplish in a short time) perhaps it would be sufficient to get just one fusion protein working. We need to prove that the enzyme part is actually getting out of the cell, so we must demonstrate that some substance which cannot enter the cell is nevertheless being degraded.

A fairly easy test would be to use a fusion of amylase to pVIII, and assay for starch degradation, which is very easy. There is no amylase BioBrick (with DNA available) in the Registry, so we'd have to make it.

Example system

A simple version of the system would work as follows:

  • E. coli are grown up containing a plasmid coding for a pVIII fusion gene, i.e:
    • Promoter-RBS-LeaderSequence-Amylase-pVIII.
  • These E. coli are infected with M13.
  • They create new phage; some of the modified pVIII proteins incorporate into the capsid.

References

Some of these were only used in Version 1.0.

Other useful links


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