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Expression systems

Lecture 7. Expression systems. Designing vectors and making protein. Few slides by David Tscharke @ RSB. Lecture overview. Ingredients of an overexpression system (cloning) Growing E. coli Induction of protein expression. The ultimate overexpression system. High copy-number plasmid

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Expression systems

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  1. Lecture 7 Expression systems Designing vectors and making protein Few slides by David Tscharke @ RSB

  2. Lecture overview Ingredients of an overexpression system (cloning) Growing E. coli Induction of protein expression

  3. The ultimate overexpression system • High copy-number plasmid • Controlled by the origin of replication • Efficient mRNA production • E. coli strain with T7 RNA polymerase (faster than E. coli RNAP) • Limited protease activity • E. coli strain deficient in proteases • Efficient for heterologous genes • Additional plasmid with tRNA genes for codons rare in E. coli • Synthetic gene (50 cents per bp)

  4. Most popular expression system • Enhanced mRNA production • E. coli strain BL21(DE3) • Contains the T7 RNA polymerase in genome under control • of the lacUV5 promoter • Induction with IPTG starts expression of T7 RNAP • T7 RNAP transcribes genes under control of the T7 promoter • Strain is deficient in lon and ompT proteases • Immune to bacteriophage 21 • “T7 vector” • Plasmid with gene of interest preceded by the T7 promoter • and followed by the T7 terminator • High copy-number plasmid • Helpful but less effective than expression of T7 RNAP

  5. T7 RNAP versus E. coli RNAP • E. coli RNAP • 459 kDa • 6 subunits (a2bb’ws) • s factor dissociates after initiation • Transcribes from many different promoters • T7 RNAP • 99 kDa • Single polypeptide chain • Extremely promoter specific EM of E. coli RNAP on DNA T7 RNAP with DNA

  6. E. coli promoters Sense sequences of selected E. coli promoters E. coli RNAP binds to the -35 and -10 regions Transcription starts at the initiation site

  7. T7promoter • Small binding site • T7 RNAP binds with very high affinity • Not recognized by E. coli RNAP Pattern required by T7 RNAP to function Schneider & Stephens, Nucl. Acids Res. 18, 6097 (1990)

  8. RNAP transcribes until it meets a terminator

  9. E. coli terminator • 1) GC-rich hairpin in mRNA, followed by 7-10 U’s “transcription bubble” site for incoming NTP • 2) In half the cases, by Rho factor • Helicase that binds to mRNA • 80-100 nucleotide recognition sequence

  10. T7terminator • GC-rich hairpin in mRNA, followed by 7-10 U’s • e.g. in gene 10 (coat protein) of the T7 bacteriophage: AACCCCTTGG GGCCTCTAAA CGGGTCTTGA GGGGTTTTTT G <<<<<<< < << >> > > >>>>>> • Non-perfect base pairing in the hairpin is OK • E. coli terminators work fine • Actually, no terminator often works too…

  11. Ribosome binding site (Shine-Dalgarno) • Ribosome binding site (RBS) • About 10 nucleotides prior to AUG start codon • Complementary to 16S rRNA of the ribosome • Promotes binding of the ribosome to the mRNA

  12. Ribosome with mRNA Showing the rRNA of the 30S subunit after stripping most proteins Shine-Dalgarno helix involving 16S RNA

  13. A real-life vector T7 terminator Resistance gene bla: “b-lactamase ampicillin” • Perfect insert has • NdeI site at 5’ end • EcoRI or HindIII • site at 3’ end T7 promoter RBS Start codon Neylon et al. Biochemistry 39, 11989 (2000)

  14. Controls for cloning • A typical cloning experiment has: • Several steps • Takes several days • You can’t easily check each step • When things go right, it’s quick • Tempting to forget about controls • What you can test: • The quality of enzyme function (only sometimes) • Can always test enzymes • Can’t always test them with specific DNA • The competency of your cells for transformation • Re-ligation of vector

  15. The most important control! • Vector only control • Even if you do nothing else… Full protocol without insert + transform • Tells you how many colonies can come from the vector alone = ‘background’ • May be re-ligated vector • May be some vector that didn’t get cut (need only pg) • May be DNA contamination

  16. What is ‘cloning strategy’? • There are often several ways to make a clone • These are the different ‘strategies’ • Each strategy has strengths and weaknesses • A strategy must go all the way • Can be considered to have five main steps: • Prepare insert (may include adding RE site using PCR) • Prepare vector • Ligate • Transform • Screen

  17. Summary I • Overexpression vector needs • promoter and terminator for transcription • ribosome binding site (Shine-Dalgarno) for translation • Using PCR to generate an insert with easy-to-clone ends (e.g. suitable restriction enzyme sites) is a very versatile method • But beware of the error rate of PCR • Never forget the ‘vector alone’ control in cloning

  18. A bit of history… Stanley Cohen • Herbert Boyer These men made the first molecular clone In which decade did they do it? Did they get a Nobel prize?

  19. A bit of history… Stanley Cohen • Herbert Boyer 1973 No Nobel prize Herby got filthy rich…

  20. Protein overexpression Induction with IPTG Autoinduction Cell-free

  21. E. coli – rich medium • Luria-Bertani broth (“LB”) • Industry standard • Made from • Tryptone (peptides) • Yeast extract (water soluble • fraction of self-digested cells • - vegemite) • NaCl • Usually also includes • Vitamins • Trace elements (metals) • Autoclave at 121 oC • to sterilize Tryptone is made from casein (milk protein) by digestion with trypsin.

  22. E. coli – minimal medium • M9 minimal medium • For labelling with expensive isotopes (15N, 13C, 2H) • Defined carbon source • e.g. glucose, glycerol, acetate, etc. • Defined nitrogen source • NH4Cl • Defined salts • NaCl, Na2HPO4, KH2PO4, MgSO4, CaCl2 • Autoclave at 121 oC to sterilize

  23. Growth curve • The cell density is measured by absorption of light at 600 nm • wavelength (optical density, “OD600”) • OD600 is directly proportional to the cell density Growth curves of different E. coli strains Maximal ribosome concentration Induce at this point plateau { exponential growth http://www.vli-research.com/silantes_labeled.htm

  24. Induction Start of protein overexpression • lac operon controls expression of T7 RNAP in E. coli BL21(DE3) • lac operon is a specific sequence of DNA • BL21(DE3) expresses a low level of lac repressor • lac repressor is a tetrameric DNA-binding protein • lac repressor binds to lac operon • Silencing the following gene (competition with RNAP) • IPTG binds to lac repressor • Binding causes a change in relative orientation of lac repressor • molecules, abolishing cooperative binding to the DNA • RNAP gains access to promoter when lac repressor leaves

  25. Lac repressor, lac operon, IPTG Isopropyl β-D-1-thio-galactopyranoside lac operon IPTG binding site Part of the other dimer in the lac repressor tetramer lac repressor (dimer)

  26. Autoinduction Method by F.W. Studier (2005) Protein Expr. Purif. 41, 207. Based on ability of certain media to induce protein expression in E. coli when cells reach saturation Once glucose has been used up, lactose in the medium is converted to allo-lactose that releases lac repressor Auto-induction can be regulated by adjusting glucose/lactose levels in media No need to monitor OD600 2-3 times higher OD600 can be reached Protein expressed while you sleep!

  27. Cell-free protein synthesis • Using the E. coli cytosol to make proteins • strip E. coli of its cell wall by shearing (pushing cells through • a small pore turns them inside-out) • Spin down genomic DNA and cell debris • Place cytosol in a dialysis bag ~ 1 ml, contains all soluble E. coli enzymes ~ 10 ml, contains ATP, nucleotides, amino acids Add DNA, T7 RNAP, tRNA to reaction volume – get protein!

  28. Cell-free protein synthesis • Typically 1 mg protein/ml reaction mixture • Good for • Proteins toxic to E. coli • Membrane proteins (detergents can be added to solubilize • the proteins as they are made) • Proteins from expensive labelled amino acids (because the • proteins are produced in a small volume and the natural • metabolism is defunct) • Fast • Proteins can be made from linear PCR-amplified DNA in • a few hours

  29. Summary II • T7 overexpression systems are the gold standard • Induction with IPTG • Classical • Autoinduction • Lazy • Cell-free • Speedy

  30. The last word… Murphy’s law “If anything can go wrong, it will” Stapp’s paradox “The universal aptitude for ineptitude makes any human accomplishment an incredible miracle.”

  31. Prac this afternoon in T4: 2 pm sharp • Lab coat and safety glasses • Risk assessment • An exercise book that will be your Log book • Read the first day of the prac and prepare a one page flow chart of the day’s experiments • Team up in pairs

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