Team:INSA Toulouse/contenu/lab practice/notebook/protocols/charac recomb





in vitro recombinase characterization protocol

The different logic gates that have been designed are described in Biological Modules. We imagined a new method for characterizing the logic gates switches with recombinases. This protocol is really easy to apply, can be used to test many recombinases with many sites, avoiding the necessity for lengthy and difficult cloning steps.

For easy characterisation of the switching steps without any complicated device, we decided to use the RFP (Red Fluorescent Protein). The gene encoding the RFP protein was directly used as a reporter system, placed under the control of the different promoters of the various logic gates.
Besides the reporter system, each logic gate must be tested with the corresponding recombinase. To save cloning time, we first thought of co-transformation. However, the different tests performed were inconclusive because of the different expression levels of plasmids and incompatibility of pSB1C3 and pSB1A2 derived plasmids. We then decided to test recombination-based switches in vitro.

In vitro switch of the designed logic gates avoids the unnecessary DNA assembling steps putting the recombinases on the same plasmid as the logic gate (which is in any case dangerous as the switch can occur during the first cloning step). Recombinases can be produced independently of the logic gates, in a regular E. coli strain and using a plasmid bearing a different antibiotic resistance than the logic gate to be tested. Soluble proteins from the strain carrying the recombinases are extracted and mixed with the plasmid containing the logic gate assembled with RFP. The mix is boiled for a few minutes and further used to transform E. coli (selecting for the antibiotic resistance of the gate, hence the plasmid containing the recombinase is not selected). The bacteria colour gives the result after culture.

With this protocol, we have been able to test in due time (two week before the wiki freeze!!) the different recombination based logic gates. The results are available in the Results part
. This protocol can be generalized to any characterization with recombinases with a possible amelioration of the specific recombinase buffer.

Recombinase overexpression and extraction

  1. Pre-culture overnight of the strain containing recombinase plasmid at 37°C.
  2. Culture at 37°C for 4 hours.
  3. Centrifuge the culture for 3 min at 13000 rpm.
  4. The cell pellets were resuspended in 600µl Rec buffer 1 (for Bxb1 and FimE recombinases) or Rec buffer 2 (for Tp901 and PhiC31 recombinases).
  5. Sonication of the cells for three bursts of 30s.
  6. Centrifugation for 15 min at 13000 rpm, 4°C, the supernatant containing proteins (recombinases) was collected and incubated on ice.

  • Rec buffer 1 : 20 mM Tris, pH 7.5, 10mM EDTA, 25mM NaCl, 10mM Spermidine, 1mM DTT and 0.1g.mol-1 BSA.
  • Rec buffer 2 : 20 mM Tris, pH 7.5, 1mM EDTA, 1M NaCl.
  • Notes : For overexpression of Tp901 and Bxb1 from Dual Controller plasmid of Bonnet, 20 ng/ml of aTc and 1% of arabinose were added to the culture in order to activate the pTetO and the pBAD promoters.

in vitro recombination test Bxb11 recombinase

  1. Recombination reactions were assembled on ice with 100 ml2 of Bxb1 recombinase extraction solution and 20 ml of solution containing Rec buffer 13 and 60 ng4 the gate plasmid.
  2. Reactions were incubated at 37°C5.
  3. Taking sample (20 µl) for different time from 15 min to 5 hours6 and heat inactivated at 75°C for 10min.
  4. Reaction taking samples were transformed into E.coli K12-DH5.1.
  5. Spread out the cells on LB agar plate containing the antibiotic to which the gate plasmid contains the resistance gene.
  6. The plate was incubated overnight at 37°C5.
  7. In order to RFP expressed can detected by human eye, the plate was exposed after in room temperature for 16 hours up to 32 hours.

The same protocol is used with different conditions depending on the recombinase(s) used:

1 Recombinase(s) tested 2 Buffer 3 Protein volume (mL) 4 Plasmid DNA (ng) 5 Incubation Temperature (°C) 6 Incubation Time
Bxb1 60 Buffer1 100 37 15min to 5h
Tp901 100 Buffer2 100 37 3h to 10h
Bxb1 & Tp901 100 each Buffer2 220 37 3h to 10h
PhiC31 100 Buffer2 100 30 3h to 10h
FimE 100 Buffer1 100 37 3h to 12h
PhiC31 & FimE 100 each Buffer2 220 30 3h to 12h

See protocols references