spacerThis year our iGEM team tested and applied the RDP assembly method by Synbiota. We used it to assemble the
polycistronic methanol uptake plasmid and to design a
monocistronic diversity library to screen for a more efficient methanol uptake plasmid.
RDP parts
RDP parts can be assembled successively to build new devices or whole new circuits with unique functions. The sequence of every RDP part begins and ends with a 4 bp long 5' overhang.
There are four differnet kinds overhangs:
X: 5'- GATG -3'
X': 5'- CTAC -3'
Z: 5'- CGGC -3'
Z': 5'- GCCG -3'
As one can easily notice, X and X' as well as Z and Z' are homologous.
A RDP part can have one of two possible orientations:
X - Z' or Z - X'
Usually, you can convert a certain sequence into a RDP part by PCR amplification. You have to use forward and reverse primers with extension containing BsaI restriction sites prior to the overhang sequence you want to introduce.
In the end, the 4 bp overhangs arise from a BsaI digest of the amplification product.
RDP parts with a length of <100 bp can be created by annealing synthesized oligos (HPLC purification recommended).
Assembling the parts
But how are RDP parts joined together to form whole new circuits? Synbiota's RDP assembly kit provides so called anchors. Anchors are linear DNA fragments, contain an antibiotics resistance gene and are bound to magnetic beads. At their downstream ends, anchors have either X' or Z' overhangs.
You can add any desired RDP part with the appropriate overhangs to the anchors that were diluted in an eppi. Matching 4 bp overhangs will be joined by T4 ligase in the reaction mix.
By applying a magnet to the eppi, you can collect all magnetic beads with the bound DNA and simply discard the used reaction mix. After washing and diluting the magnetic beads again, you can add your next RDP part. After assembling the RDP parts, you'll have to add a so called "cap". It is also provided in Synbiota's assembly kit and contributes an origin of replication.
Finally, anchor and cap can be joined to form a circular plasmid. By choosing a certain anchor and cap for the assembly you determine the resistance marker and copy number of the resulting plasmid.
RDP plasmids can again be digested with BsaI and NotI to extract the assembled RDP parts as one composite RDP part.
Experiences
We experienced the RDP assembly method to be really easy to handle. Both the in silico
work to design new RDP circuits and the efficiency of the method itself convinced us.
Thus, RDP assembly can be a suitable alternative to other assembly methods like Gibson Assembly
or CPEC.