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The benefit of vector assembly in yeast

Introduction

iGEM Team Frankfurt 2012 successfully used a relatively new methode for vector assembly in yeast called Gap Repair Cloning. It is a more and more established methode for efficient, fast and error-free construction of plasmids based on the homologous recombination system of Saccharomyces cerevisiae (common yeast). Naturally yeast uses this process to repair DNA double strand breaks which are one of the most dangerous and life-threatening damages of the DNA for a cell. Therefor this eucaryotic microorganism has developed a few enzymes which have the ability to repair a broken DNA double strand by pairing it with a very similiar DNA region (typically on the homologous chromosome).

Using gap repair cloning a series of linear, successive DNA fragments with homologous overlaps to the respectively following fragment can be transformed in only one step into a yeast cell. After that the micoorganism recombines all fragments in the predetermined, specific order to the final targeting vector. The advantage is that up to eighteen and more successive DNA fragments can be assembled in a single transformation. Also only one restriction enzyme for linearization of the plasmid is needed.

For these reasons iGEM Team Frankfurt thought that gap repair cloning is usefull tool for next iGEM generations. Therefore we developed a standardized methode that describes a new way of assembling BioBrick devices in a desired order to a targeting plasmid using homologue recombination system of yeast. It is called Yeast BioBrick Assembly (YBA). YBA standard only needs one restriction enzyme and a standardized selection of primers and promotors/termintors. It is a continuation of the BioBrick standard and compartible with all BBF RFC 10 parts. Additionally it can be adapted by specific primer design to all other BioBrick standards. In the following we focuse on assembly of yeast expression vectors by using YBA methode. However it also can be used for E.coli vector design or assembly.

Homologue Recombination System of Yeast

There are many endogenous and exogenous factors (for example reactive oxygen-species, ionizing radiation, chemicals and failing of DNA binding enzymes (e.g. collapsed replication forks)) which causes DNA double strand breaks. For the cell this is the most dangerous DNA damage because even if it occurs in rather unimportant regions the cell will not survive the next cell cycle. That's the reason why yeast possesses highly active enzymes which have the ability to repair a broken double strand by pairing it with a very similiar DNA region (typically on the homologous chromosome). This process is called homologous recombination. Using the gap repair method this natural process can be exploited for the construction of large cloning vectors in yeast.

Design of DNA fragments for gap repair cloning

For the formation of a cloning vector the first fragment is a yeast-E.coli shuttle plasmid which is linearized by an appropriate restriction digest. A shuttle plasmid is a plasmid which is stable both in yeast and in Escherichia coli. The first fragment of the insert has to possess an homologous overlap to both the wished insertion site on the plasmid and to the beginning of the second fragment. The end of the second fragment has to possess an overlap to the beginning of the third one and so on. At least the end of the last fragment of the insert again has to possess an overlap homologous to the second insertion site on the plasmid.

At the lab of our instructor up to eighteen single fragments were assembled in a single transformation. Another advantage of the method is that no scars are left between the inserted fragments. Assembly of fragments to joint genes is possible. Restriction enzymes only have to be used once for linearization of the shuttle plasmid.

Example from our project

The overlap sequences at the beginning and the end of each fragment are 20 bp long and homologous to the respectively previous and subsequent fragment. The overlaps to promoter and terminator on the plasmid are longer (40 bp) because the plasmid does not contain any overlaps to the first and last insert fragment. Thus the homologous region between every fragmentpair is always 40 pb long.

In our project we have constructed a plasmid for overexpression of three genes of the Mevalonate pathway. Therefore we used synthesized fragments of the mentioned three genes, two promoters, two terminators (both from yeast) and a yeast expression plasmid which already contained one promoter, one terminator and a gene for uracil synthesis. Moreover we used a mutant strain where this uracil gene is deleted. So we needed the uracil gene on the plasmid as a marker for selection of the transformants. Via PCR we assembled appropriate homologous overlaps to our fragments using primers containing these overlaps. By that we were able to arrange our fragments in the order shown in the picture on the right.

The DNA of all fragments is transformed together into competent yeast cells. The transformants were selected on synthetic medium lacking from uracil (only transformants should be able to grow). After preparing the plasmid from the transfomed yeast clones it can be transformed into Escherichia coli and gained from there in large amounts for further use.

The DNA sequence of the complete expression vectore is shown in the following file: Media:DNA_Sequence_of_Plasmid_for_Overexpression_of_Mevalonat_Pathway.txt‎.

Yeast BioBrick Assembly - Gap repair cloning for iGEM

Example of BioBrick Assembly via gap repair cloning with YBA standard. First primers are assembled to the respective gene via PCR. The primer overlap to suffix or prefix is about 20 bp. Now the assembled genes have homologous overlaps to the respective promoters and terminators of about 40 bp length. In a yeast transformation the shuffle plasmid, a ligated terminator-promoter part and the assembled genes are put into the yeast cells. They put all parts together via homologous recombination to form the complete vector.

Variable assembly of terminator-promoter parts via gap repair cloning.

The BioBrick cloning standards used in the Parts Registry and in the iGEM competition are based on restriction digest and ligation. One of the main advantages of the gap repair method is to avoid this. Moreover it leaves no scars between the assembled fragments like restriction digest and religation does. Another advantage of gap repair cloning is it´s heightened time effiency when a large amount of fragments shall be assembled. Furthermore the expensive use of restriction and ligation enzymes can be reduced significantly. For these reasons gap repair cloning promises to be a useful tool for future iGEM teams. The problem is that the common Biobrick standards are useless by now with regard to gap repair cloning. The idea of YBA is now to design a new standard for assembly of yeast vectors based on standardized PCR primers. YBA is compartible to common BioBrick standards and allows gap repair cloning.

Although the restriction sites of the Biobrick pre- and suffix are unimportant in our context, the biobricking of genes leads to possiblity to amplify all Biobricks with the same prefix and suffix type (in our case we focus on BBF RFC 10) with suitable PCR primers. There are already some PCR primers in the Parts Registry which anneal at the pre- or suffix sequence. By adding an specific non annealing sequence to the primers a desired overlap to other fragments can be produced on every Biobrick device. These primers which consist of an annealing sequenz to either prefix or suffix and an specific overlap are the stardardized PCR primers for YBA method.

The further idea would be to create DNA fragments suitable for gap repair cloning. It would be great if the assembly and cloning of genes in yeast become simpler than the cloning by restriction and ligation. In the following we want to outline the basic idea. Any yeast expression plasmid has a similiar ordered insert region. Depending on the number of genes on the final plasmid the insert region consists of several repeats of the general scheme promoter-gene-terminator. Gap repair suitable DNA can simply be created by choosing promoters and terminators of yeast and the desired genes from the parts registry. Now every gen has to be amplified with YBA standard primers which contain homologue DNA overlaps to the end of a chosen promoter and the beginning of a proper terminator. With such PCR products it is possible to do the homologue recombination in yeast like it is shown on the first picture on the right.

Since any Biobrick (from the same standard) can be amplified with the same primers it is sufficient to construct one standard primer for every promoter, overlapping with the end of the promoter, and one for every terminator, overlapping with the beginning of the terminator. We designed one standard primer compartible to BioBrick RFC 10 for any of the yeast promoters and terminators we used in our project. Of course the annealing sequence of every standard primer can be adapted to all other BioBrick RFCs.

The connecting terminator-promoter fragments between the BioBrick devices shall not have BioBrick prefix or suffix at their ends because this could lead to incorrect assembly of targeting vector and reduces the efficiency of gap repair cloning.

After all there are two ways of connecting the terminator of one gene with the promoter of the following gene. The first one is to design specific combinations of ligated terminator-promoter parts (shown on the first picture). At the vector assembly you just have to transform the linearized plasmid, the amplified genes with homologue overlaps and the terminator-promoter parts to get the targeting vector. The advantage is that you need a lower number of DNA fragments which increases the efficiency of gap repair cloning. We designed a few of this terminator-promoter parts for the registry. The disadvantages are first that you are not able to choose all possible combinations because they are too many. Secondly you have to amplify the parts out of the BioBrick plasmid anyway. Therefore you must use a forward primer annealing behind the prefix and a reverse primer anneling in front of the suffix. This leads to the second way.

The second way allows much more possibilities of combining terminators and promoters (shown on the second picture). Here you can choose BioBricks of single terminators and promotors from the parts registry. First you amplify the terminator with two specific primers annealing behind the prefix and previous to the suffix. The next step is amplification of a suitable promoter which comes after this terminator. There you take one primer annealing at the prefix. This primer has an additional homologue overlap to the previous terminator to connect both at gap repair cloning. The second primer is annealing in front of the suffix.

In the end you transform all amplified genes, promotors and terminators and the linearized plasmid into yeast cells to get the targeting plasmid.

Whenever you want to assemble a high number of BioBrick devices in a specific order to a targeting vector you can use standardized YBA methode for an easy, effective and cheap assembly.

Yeast Vector Assembly Kit