Combining Fusion of Cells with CRISPR-Cas9 Editing for the Cloning of Large DNA Fragments or Complete Bacterial Genomes in Yeast.

The genetic engineering of genome fragments larger than 100 kbp is challenging and requires both specific methods and cloning hosts. The yeast Saccharomyces cerevisiae is considered as a host of choice for cloning and engineering whole or partial genomes from viruses, bacteria, and algae. Several methods are now available to perform these manipulations, each with its own limitations. In order to extend the range of yeast cloning strategies, a new approach combining two already described methods, Fusion cloning and CReasPy-Cloning, was developed. The CReasPy-Fusion method allows the simultaneous cloning and engineering of megabase-sized genomes in yeast by the fusion of bacterial cells with yeast spheroplasts carrying the CRISPR-Cas9 system. With this new approach, we demonstrate the feasibility of cloning and editing whole genomes from several Mycoplasma species belonging to different phylogenetic groups. We also show that CReasPy-Fusion allows the capture of large genome fragments with high efficacy, resulting in the successful cloning of selected loci in yeast. We finally identify bacterial nuclease encoding genes as barriers for CReasPy-Fusion by showing that their removal from the donor genome improves the cloning efficacy.

CReasPy-Cloning: A Method for Simultaneous Cloning and Engineering of Megabase-Sized Genomes in Yeast Using the CRISPR-Cas9 System.

Over the past decade, a new strategy was developed to bypass the difficulties to genetically engineer some microbial species by transferring (or "cloning") their genome into another organism that is amenable to efficient genetic modifications and therefore acts as a living workbench. As such, the yeast Saccharomyces cerevisiae has been used to clone and engineer genomes from viruses, bacteria, and algae. The cloning step requires the insertion of yeast genetic elements in the genome of interest, in order to drive its replication and maintenance as an artificial chromosome in the host cell. Current methods used to introduce these genetic elements are still unsatisfactory, due either to their random nature (transposon) or the requirement for unique restriction sites at specific positions (TAR cloning). Here we describe the CReasPy-cloning, a new method that combines both the ability of Cas9 to cleave DNA at a user-specified locus and the yeast's highly efficient homologous recombination to simultaneously clone and engineer a bacterial chromosome in yeast. Using the 0.816 Mbp genome of Mycoplasma pneumoniae as a proof of concept, we demonstrate that our method can be used to introduce the yeast genetic element at any location in the bacterial chromosome while simultaneously deleting various genes or group of genes. We also show that CReasPy-cloning can be used to edit up to three independent genomic loci at the same time with an efficiency high enough to warrant the screening of a small (<50) number of clones, allowing for significantly shortened genome engineering cycle times.