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Protocol Cell-free expression

Preparation of a Lysate for Cell-free expression

Material: see below
Preparation: 2-3 days

To achieve high levels of protein production, the BL21(DE3) strain was chosen, which carries the gene coding for a T7 RNA polymerase as a lambda prophage within its chromosome, which is inducible by addition of IPTG
As the system is later to be used for expression of synthetic gene constructs of varying natural hosts, the bacteria need to be able to express so-called “rare codons”: AGA, AGG, AUA, CUA, GGA, CCC, and CGG are used less frequently in E. coli as they are e.g in human cells
Cells were transformed with the pRARE2 plasmid, taken from Rosetta2 competent cells. The pRARE2 plasmid encodes for these rare codon - tRNAs

Prepare an overnight preculture in 10 ml medium in a 100 ml Erlenmeyer-flask @ 37°C, 250 rpm
inoculated in 1000 ml Terrific Broth in a 2000 ml chicanery-flask at 37°C, 150-180 rpm
cells were grown until reaching OD600 = 0,8
The expression of T7 RNA polymerase then was induced by adding 0.5 ml of 1 M IPTG to the medium
further growth until OD600 = 2.5, then cells were put on ice
cells were pelleted at 6000 g at 4°C for 30 minutes in a fixed-angle rotor
supernatant was decanted and the cells were washed two times in buffer A
Again, the cells were pelleted at 5000 g for 20 min at 4°C and afterwards resuspended in 1.3 ml of buffer B
Using a french press with a 1“ diameter piston, the cells were lysed at 1000 psi (≈ 6.9 MPa)
About 7.5 ml of lysate was obtained after pressing.

obtained lysate is pelleted via ultracentrifugation at 30,000 g for 30 min at 4°C
The resulting extract is mixed with the preincubation buffer in a ratio of 10:1 (lysate:buffer)
preincubated lysate is then transferred to a 3.5 kDa dialysis tubing and desalted by dialysing it against buffer C at 4°C for 90 minutes
after renewing the buffer, once more over night at slow stirring
the lysate is ready for use and, after shock-freezing in liquid N2, can be stored in small ali-quots at - 80°C.

The Preincubation step takes care of what is called the „run off“ step, a step imple-mented in order to eliminate endogenous cellular mRNA. The removal of endog-enous mRNA bound to ribosomes/polysomes is accomplished by finishing their synthesis via the added PEP, pyruvate kinase and ATP at 37°C. The residual endogenous mRNA will then subsequently become degraded due to the high RNase content of the extract. This step is precautionally performed in the dark, because some components are slightly light sensitive.
The lysate can be frozen again after use, but looses effectivity

Reaction is adapted from the European Molecular Biology Laboratory (EMBL), Heidelberg, Germany

@@ Line 91: / Line 91: @@
 <li class="level1"><div class="li"> To achieve high levels of protein production, the BL21(DE3) strain  was chosen, which carries the gene coding for a T7 RNA polymerase as a lambda prophage within its chromosome, which is inducible by addition of IPTG</div>
 </li>
-<li class="level1"><div class="li"> As the system is later to be used for expression of synthetic gene constructs of varying natural hosts, the bacteria need to be able to express so-called “rare co-dons”: AGA, AGG, AUA, CUA, GGA, CCC, and CGG are used less frequently in E. coli as they are e.g in human cells</div>
+<li class="level1"><div class="li"> As the system is later to be used for expression of synthetic gene constructs of varying natural hosts, the bacteria need to be able to express so-called “rare codons”: AGA, AGG, AUA, CUA, GGA, CCC, and CGG are used less frequently in E. coli as they are e.g in human cells</div>
 </li>
-<li class="level1"><div class="li"> cells were transformed with the pRARE2 plasmid, taken from Rosetta2  competent cells. The pRARE2 plasmid encodes for tRNAs</div>
+<li class="level1"><div class="li"> Cells were transformed with the pRARE2 plasmid, taken from Rosetta2  competent cells. The pRARE2 plasmid encodes for these rare codon - tRNAs</div>
 </li>
 </ul>